fatal attraction
1. In a romantic relationship, an attraction to the very qualities in a person that ultimately lead to the relationship falling apart. John had a fatal attraction to the wild, uninhibited life Cynthia led, but she cared more about "living free" than having a functional relationship. I was completely enamored by his self-confidence and charm, but it was just a fatal attraction to a man who turned out to be a complete narcissist.
2. An intense and obsessive infatuation with another person, especially by a spurned lover, that might lead to harassment or violence. A: "I broke up with Chris over two months ago, but he's still constantly checking in on me and even started threatening this new guy I started seeing." B: "Whoa, talk about a fatal attraction."