I would not automatically jump to the worst case conclusion that he's definitely exhibiting serial killer traits. Like, if he views them as pest animals, that's not exactly normal and healthy, but it isn't clear evidence of something much worse either.
Ideally, he should probably talk to a professional to help him deal with it in a healthy way, if he isn't already. Telling your friend and/or their parents that you're worried about him, and quoting specific things he's said, is probably a good place to start.
Would it have actually worked though? The access Hollywood tapes came out during that election, Trump's behavior of entering the dressing rooms of Miss Teen USA were well reported on, and neither really hurt him. Maybe there's a chance it would've changed the outcome, but it's hard to imagine that chance is large.