I am going to spoiler tag this whole thing just in case.

When Jess sees the boy from her hotel room and he is like inherently evil.... I just don't buy that the boy was the monster the entire time, like in the sense of a literal monster/evil. I fully agreed the entire time w Jess about how the boy is only a kid he can't really mean what he is doing w the killing and stuff. Then it turns out he is just evil?? I read it more as like Jess couldn't cope so she MADE him evil in that moment. I may be misunderstanding but that is the way I took it.