The author of ZeroMQ wrote a book on recognizing people like this, and more importantly how to get yourself out of prison they’ve put you in. The action plans have helped me. The lucky ones in the NYmag article here did many of the things he recommends. Free PDF here, look for the hard to find “read” button. https://www.gitbook.com/book/hintjens/psychopathcode/details
I once had a psychopath like this turn up in my life. Like a lot of these stories, it escalated slowly, one creepy / inappropriate thing at a time. There really is a frog in hot water effect in play here. If you haven't experienced it, it might be hard to understand why someone let themselves be in that situation at all.
What I don't understand, however, is why people don't deal with it decisively once they realize what's going on. When the person harassing me tried to cross an un-crossble line, I beat them within an inch of their life and then drove them to the hospital. I stuck around to talk to the police if they showed up, but they were never called.
I stopped in the hospital the next day to make sure this person wasn't going to bother me again. While I don't revel in this -- I see it more as more instinctual than intellectual -- the person literally shit in their hospital bed when they saw me in the doorway. Putting someone in a position where they have lost control has a profound psychological effect -- it works on psychopaths as well as their victims.
Yes, there was a chance that I could have gone to jail or this person could have tried to retaliate. But I accepted those possible outcomes and did it anyway. People risk their lives / well being all of the time for things a lot less important than their lives and well being (BASE jumping anyone?). I don't regret it one iota and I'd do it again.