Death managed to entangle Susan in his scheme to rescue the Hogfather by using reverse headology. He explicitly forbade her to get involved, which someone as headstrong and defiant as Susan was bound to ignore.
“She’ll try to find out what this is all about, you know,” said
Albert.
OH DEAR.
“Especially after you told her not to.”
YOU THINK SO?
“Yeah,” said Albert.
DEAR ME. I STILL HAVE A LOT TO LEARN ABOUT HUMANS, DON’T I?
He knew that Susan would step up to the mark and deal with Teatime because, frankly, his actions toward the children of the world are monstrous (subjecting them to magical mind control) and Death knows that Susan likes children, as evidenced by her career choices. She's the perfect weapon to aim at Teatime and his crew.
“So now we know where the teeth were,” she said. “They must’ve taken them out of everywhere and carried them downstairs.”
“What for?”
She sighed. “It’s such old magic it isn’t even magic any more,” she said. “If you’ve got a piece of someone’s hair, or a nail clipping, or a tooth—you can control them.”
The oh god tried to focus.
“That heap’s controlling millions of children?”
“Yes. Adults, too, by now.”
“And you…you could make them think things and do things?”
She nodded. “Yes.”
When she finds out what her adversary is actually doing, she's not even incandescent with rage, she's cold and emotionless which is, for Teatime, far worse*
* Fatally worse, as a matter of fact.
