You don't need ill intent to do wrong.

Do I need to have ill intent to just pocket an item at a store because my hands are full and then forget to pay for it? Do I need ill intent to miss noticing a cyclist when turning and clip him with my car? Do I need ill intent to clean my balcony and accidentally drop the flower pot onto someone's head? Do I need ill intent to drive with a broken headlight to get a new one?

Just that series of examples shows that damaging others or their property isn't done with ill intent but sloppiness, oversight, or clumsiness. Accidents are bound to happen. Violations of law that don't directly impact the property or life of another will happen. Those violations, in turn, will create some accidents or make them worse, so there is a requirement to police such violations. The saying that safety regulations are written in blood is mostly true, and it requires no intent for most of the acts. Take a look at these two examples:

1. The box you put into the hallway because it was too heavy? It's right in front of the door of the flat that is burning and blocking it. Did you have ill intent toward the people inside? No. But it is in the way.
2. There's a lathe setup that you just figured out how to make two parts at the same time and reduce your setup times, but it easily mangles your hands and arms if you so much as touch the stock with the tool in the wrong way. Did you have ill intent to yourself or any operator of your setup? No, but the setup is dangerous anyway.

A lot of things happen that police can and will investigate. Accidents or, when people don't feel they harm someone by doing something is a large load already, but add to that the general enforcement of other laws and ordinances. The box-and-fire example? Stopping the box there longer than strictly necessary violates the fire code, and a fine for that might be in order.

Policing power also is, as JBH stresses correctly, far wider. Not just enforcing the laws as enacted, but also the judgements need enforcement.

Say, Alice owes Bob money and cannot pay it back. She didn't have ill intent toward Bob when she borrowed the cash or spent it. She thought she'd earn back enough to pay Bob at a future date. But she had a business plan that did not come to fruition and has no cash when the loan is due. Bob, in turn, has no ill intent toward Alice as a person, but he is entitled to his money back under the loan contract. So Bob sues, and the court decrees that some of Alice's property is to be given to Bob to compensate him. The police will be there at the transfer to ensure everything is all right.

The Joy of Worldbuilding: Police Power

You're asking the wrong question, probably because you've never thought about what "police power" is.

Police powers are the fundamental ability of a government to enact laws to coerce its citizenry for the public good, although the term eludes an exact definition. The term does not directly relate to the common connotation of police as officers charged with maintaining public order, but rather to broad governmental regulatory power. Berman v. Parker , a 1954 U.S. Supreme Court case, stated that “[p]ublic safety, public health, morality, peace and quiet, law and order. . . are some of the more conspicuous examples of the traditional application of the police power”; while recognizing that “[a]n attempt to define [police power’s] reach or trace its outer limits is fruitless.” (Source)

No government can exist without the intrinsic threat of violence. That's an ugly way of saying it, but there it is. Even a polite and congenial agreement between neighbors must carry with it an implicit "or else..." lest chaos and unpredictability reign. Where there is order of any kind, there must also be the possibility of punishment for upsetting that order. The real questions are "who?" and "how much?"

Your friendly neighborhood food inspector is a copper!

When people hear the phrase "police," they nearly always think of uniformed people who beat the crap out of innocent bystanders during a riot. Or something like that. An organization that's as obviously corrupt as it is necessary, right?

The truth is that there are a LOT of "police" out there. Building inspectors, health inspectors, compliance officers, guards... just to name a few. Anyone who has the ability to force someone else to an action (such as closing a restaurant, stopping construction, turning away the unwanted...) is exercising police power.

Now, the question of police power can get really complicated. Who has the "right" in an organized society to hire bodyguards? If the laws don't permit it, the answer is "nobody." (Laws like that would quickly lead to new elections or a revolution. Same thing, different by extent.)

Therefore, the question isn't, "what role would the police play in my society?" They'd play the same role they play in every society whether it's the tribal leader's burly son or a privateer. Here are your questions:

1. What is the source of authority? Where there are laws, there are punishments for breaking the laws (otherwise they're not laws, just polite suggestions). Those laws must be based on something, some kind of agreement among the governed about how government (and, by extension, society) will be structured. You say you have laws, if only a few. Where'd they come from? Who decided to write them and why? Who agreed to them and why? All this leads you to your world's structure of authority.

2. How much authority do the police have? Granted, where there's authority there is almost always someone who will abuse that authority. Let's ignore that uncomfortable truth of human existence. Are your proverbial drunk legislators willing to let the police throw them in jail for driving under the influence? Punishment is in many ways just as complex as the laws themselves. Just what can the police do and when can they do it? What does "illegal search and seizure" mean when the words are brought into a court room? Law generally, and punishment specifically, can be contentious as the "we don't want you to do that!" people begin arguing with the "but I want to do that!" people.

And to make a point, here in the U.S. (assuming we're in one of the areas where it's been implemented better-than-average) "police power" is broken into three groups:

• The police, who don't have the authority to punish anybody, but who do have the authority to bring force to bear for the sake of both order and compliance.

• The judiciary, who has the authority to assign punishment, but does not have the authority to bring it to pass.

• Correctional facilities (wither the individual hangman or a large, complex prison system) are the people charged with actually effecting the violence. It's one of the reasons why, traditionally, those who work in this end of the "order" process have been shunned.

This can be as small as a single person or two (The U.S. Old West was famous for a two-person solution: the copper serving as both police and correctional facility and a judge) or as large as today's complex judicial systems, which range from small-town police and county sheriffs to various three-letter-acronym organizations and separate departments of Justice and Corrections.

So, what's my answer?

Of course you'll have "police." Whomever is responsible for ensuring those laws are obeyed and whatever punishments attached are meted out, they're your police. Regardless what name you give them.

Police power: the fundamental ability of a government to enact laws to coerce its citizenry for the public good.

No government, no police.

Government? Police.

"Ill intent" is not "criminal intent"

• Ill intent is where you wish ("will" or "intend") bad stuff or harm ("ill") to befall others. Ill intent is a synonym for malice.

• Criminal intent is where you wish to perform an act which would break some rules laid down by government.

OP has suggested in comments that their definition would cover all cases of desire-driven theft. To me this implies either a misuse of "ill intent" to mean "criminal intent"; or a belief that all crime is necessarily malicious.

There is a very large set of criminal intents which are not driven by ill intent. Outside of violent crime, very little crime is malicious.

Let's list some types.

• Murder. Is euthanasia outlawed? The act of murdering someone in this situation is arguably one of the greatest expressions of empathy, so cannot reasonably be called ill intent.

• Theft. Vanishingly few acts of theft are carried out because the perpetrator wants the owner to have less stuff or to be worse off. They typically bear no ill will to the owner. Instead, they want themselves to have more stuff. Theft is an act of selfishness, not of malice.

• Bribery Nobody gives someone else money because they want that person to be worse off. Bribery is about making things easier for people, not making things harder: an act of laziness, not of malice.

• Smuggling, Drug trafficking Plenty of people smuggle things and sell drugs because they disagree with the laws. They don't see what they are doing as harmful, but as helping people.

• Protesting Nobody protests out of ill will. They protest because they want things to be better, for themselves, and usually also for others.

• Prostitution Obviously not malicious.

• Perjury Done to protect self or others. No malice to others need be intended.

• vandalism Can be malicious, but also caused by self-expression (graffiti), fiddling/playing (accidental arson), curiosity (pushing over historical boulders), etc.

• Speeding and other traffic infractions Usually laziness, lack of caution or attention, etc.

In the real world, we already have mechanisms in place to prevent us from being malicious: conscience and morality. Nobody wants to be the bad guy. So, for many types of crime, people rationalize their actions to themselves so that they don't have to bear any ill will.

Personally, if I wanted to make police unnecessary by preventing a mode of thought, I'd make it so that criminal intent is prevented, rather than ill intent. Or change the world's laws so that only violent crime is illegal.

Terrorism and Extreme Ideology

People who hold radical beliefs do not need to have ill intent to cause harm to others. They fully believe what they are doing is for the greater good. Just like how a person kills to prevent a person from murdering their family, the mind of a terrorist views what they are doing in a similar way. By killing people who do not follow their beliefs they think it will help prevent its spread and the harm it will cause. Of course this warped mindset is wrong, but unless that city's magic can also alter people's beliefs there will still be conflict.

I suspect that most readers would not even think about it if some group were described as "The Guardians", without specific details as to their duties. Maybe someone manages to get into the city but the protection fails for some reason. They may be police in the loosest sense, but probably the most inept police anywhere (due to lack of practice).

Here's a list of possible duties for your City's Police Force.

Issuing fines (take your pick for what), licensing inspections, welfare checks, executing court orders and summons to attend court, seizing property under judgement, investigating accidents and preparing reports for the courts or the coroners office to assist in apportioning blame. Monitoring foreign nationals (who may have malicious intent) since your description doesn't state whether foreigners can enter your city with evil in mind as opposed to residents. Locating and arresting felons wanted in other jurisdictions for crimes committed there and repatriating them, guarding important public buildings or persons, traffic and crowd control and emergency response.

There's more things I could add to the list but long story short? A Police force would still be required.

Based on this Evil Ward, it seems to have two main parts to it:

• No one can harbor ill intent within their minds
• Any action that might harm another to your own benefit would seem to you as repulsive

That second part, about harming another, is the dangerous part and will be the biggest determination of what triggers the revulsion.

Also, just because an idea is repulsive, does not mean that somebody won't do it anyways if they are desperate enough, or have enough ill intent to overpower the revulsion.

The Case for Police

Would there be police? Short answer, yes.

However, they are likely not be as well armed as some police forces of the world. Taking the decision to end a life, even in pursuit of your duties is harming another for your own benefit and said officer almost certainly has ill intent towards the person they have decided to strike down. Your police force will know this and either have something to bypass the spell, or more likely have to use their words more than their weapons.

The crime profile might be different in this city, but there are still disputes that would require mediation and enforcement -- noise complaints, domestic arguments, enforcement of court orders, etc. There will still be violence and thefts, but they will be lessened by the spell -- hopefully.

Possibly, given the fact that the Anti-Evil Spell curtails a lot of crime, a police force in this city might be more public service oriented and have a better overall reputation -- a trusted point of first contact for those that need help in general. These are the people that can be the launching point to get in touch with other services.

But yes, there will still be some manner of law enforcement organization. The Anti-Evil spell curtails evil, not lawlessness.

Plus, this does not get into any other government organization that enforces specific sets of rules. You know, like the group that enforces governmental order by the invocation and maintenance of the large Anti-Evil spell over the city.

On Anti-Evil

Of note is that the two halves of your statement are not necessarily mutually exclusive. You can harm somebody without harbouring ill intent against them.

For an extreme example, somebody can fell no ill intent in their murder of another because they lack the necessary mindset to feel that intent. As the murder doesn't benefit them in any way, it shouldn't trigger the repulsion of the spell on that count either.

Depending on the exact nature of the Anti-Evil spell, no surgeon could practice in the city as while they have no ill intent, technically they are doing harm in their attempt to heal (surgery) for their own benefit (a paycheck).

While I don't know the parameters of the Anti-Evil spell, it could be anything from a mild nuisance in one's life to wildly defining one's existence.

But be aware that Evil does not always mean lawless nor Good lawful. It's an interesting philosophical debate that I am absolutely unqualified to have as I did not study this in school.

Societal Change

Unstated in the question, but something else to consider is how this Anti-Evil ward would affect the city's operation itself. Ill intent and harm to another for self benefit are vague -- perhaps intentionally so -- but it does open up questions about what exactly it takes to trigger the spell and how the city, and the society born from it have to change in order to live by the dictates of the spell.

Perhaps once the effects on the society are determined, it becomes easier to figure out what kind of law enforcement forces are needed.

If they could reliably decide what is 'bad' then they would only need one law "don't do bad stuff". But things are not that simple. Other answers have pointed out that people may do bad things for many complex reasons. And the police would be required to settle disagreements.

Suppose instead the one law was "love one another".

People claimed 2% of a population practicing transcendental meditation could improve the well-being and community spirit of an entire city.

Timothy Leary's experiments with LSD gave him a powerful sense of 'belonging to a whole'. He reasoned that this could supply the feelings of empathy so lacking in profession al criminals. In experiments in prisons he claimed the people he treated had a much lower rate of re-offences.

I don't believe either. The first one could not be measured, and the second one has not been repeated (there are too many types of LSD and reactions are very personal). However, you could reasonably imagine a city with common parks and libraries, free healthcare, a meditative lifestyle, and a low level of LSD in the water, or the magical equivalent thereof, might stop anti-social crimes, and would cause people to own up to accidentally doing harm. The few that remain might be treated as mad people were treated in 'Erewhon': as people who were disadvantaged not through their own fault, and deserving of our pity and kindness (people who were ill in Erewhon were forced to hide this as being ill or old was shameful). I think you could do without police in the sense we know.

Deliberate crime is not the only thing that Police deal with.

One very common thing that they deal with is missing-persons: this could, with "ill intent", be a kidnapping. Or, it could be someone who suffers from dementia and has wandered off. It could be a hiker who has fallen down a crevice, and cannot phone for help.

And, what about animals? Are they covered by your magic field? If food is missing from your pantry, it might be a cat or a dog breaking in.

Lost Property? If you accidentally drop your wallet or purse while out-and-about, who will the well-intentioned finder hand it in to?

Accidents? If someone dies because their medicine was the wrong dosage, then who determines where the error occurred? (Especially since the medicine might have been prepared outside the city, and sent in with an unwitting patsy)

Or, just, well-intentioned disputes: two people both believe that something belongs to them. Who investigates to make the final decision?

And, finally… who arrests criminals who commit their crimes outside the city, but live there as a "safe haven"?

Who do you think provides those "magical means"?

In this city (by magical means I would happily detail if needed) there is no evil...

Whoever set up those "magical means" is the police force.

"Evil" is not something which can casually be used as a descriptor such that everyone knows what it is. Physical violence seems logical - but it eliminates most sports. Taking without permission seems logical - but it eliminates taxation. No, in order for this to work, the creators of this magical law enforcement forcefield must start from specific laws which they intend people to be prevented from carrying out. And at the point they are enforcing laws, they become the police.

Granted, they might not need to become thief-takers, and trying around catching perps. But their deeper role in law enforcement, and everything related to that including oversight, is very much going to be important.

For your own good

The other answers provided a lot of scenarios that require police, but I didn't see anyone mentioning that you'd need police to manage cases where someone honestly believes that they're doing the other person a favor. Those who send their kids to "Pray the gay away" camps don't harbor ill will, they honestly think that this is a problem. Con men argue that they're doing the person a favor by teaching them caution.

Really, you can take this a step further. Sociopaths don't do what they do because they want to hurt the other person. They do it because they enjoy doing what they do. Their victims are often just targets of opportunity, and they have de-humanized the target to the point that they aren't able to consider that person's happiness in their motivation.

Then you get the Robin Hoods, who will do harmful things in order to better society. In our world, they might harbor ill will towards their targets, but that's incidental to their motivations.

You also have cases where someone takes something because they feel they deserve it. Again, no ill will, just an adjustment of ownership.

Of course you'll need police. Law enforcement is not about keeping the peace or maintaining domestic tranquility, it's about control. Since you're building a world I must assume you, as the Prince, want control. With or without ill intent you will need cops to enforce your control.