**Neither a teacher nor a native speaker.**
I am very sure that it's people's.
When you see toilets you read such things:
s' were correct if the sentence would be:
Serial Killer: "Yeah. I enjoy taking humans' lives!"
Singular: Human -> Human's
Plural: Humans -> Humans'
People is already plural, so you just need so add an 's
s' would be correct if the sentence were.
(This is the correct version of the sentence I wrote some hours ago.)