I think the general lifespans of monsters is why attempting to kill someone is taken so casually if someone or something is being a problem. Humans don't generally live as long as most monsters do, so it becomes problematic when someone tries to kill another person.

Really, all I can think of is that she should cut off absolutely any contact with him. He only really ever seems to just make her fly into a rage-- be it because he deserves it, or because she has a Pavlovian response at this point towards him.

Though the stunting her emotional maturity thing probably doesn't help either, but I don't see how that would equate to wanting to kill someone.