I saw a film in the mid-to-late nineties about a vengful woman.

Basically at the start of the film she has (possibly) been released back into the community. Her ex boyfriend is now married with a young son. She sets about ruining his life. She murders her old shrink by drowning her in the bath, she kills a woman who lives in the same apartment as him in order to live close to him, she accuses him of assualting her and he is forced to pay a 100,000 dollar bail. At one point in the film it looks like she is going to kill his son by pushing him off a boat but stops at the last minute. She ultimately dies when the little boy sets her clothes on fire.

The Ex (1997)