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.