Macbeth Act 2, Scene 2:

Things bad begun make themselves strong by ill.

Macbeth Act 2, Scene 2:

Things bad begun make themselves strong by ill.

could actually be used as an idiom though you might raise a few eyebrows. If you want to go for an applause you might try these lines from Act 3, Scene 4:

I am in blood Stepped in so far that, should I wade no more, Returning were as tedious as go o'er.