Is it vindication or simple spite?  

Men do this too, and in about the same number. It makes absolutely NO sense to me whatsoever. Of course, I am a creature of loyalty and logic - this may have something to do with it. People who are unhappy and don't really wish to change it tend to bring others down also. Nor do they care who they hurt, it makes them feel better in some odd, senseless, destructive way. I would love to have the answer to this question. I'm afraid there isn't one, though, as I have been on the receiving end of this situation before, and tried everything in my power to figure it out. Of course, I ended up leaving the guy, and telling the woman who cause our problems that she could have him. I didn't want to be with someone who would do that anyway...people like that always repeat their behavior.