Need the name of a movie?

Dear everyone, I've been trying to find the name of a movie I watched decades ago. The main character is a construction engineer or architect, but I think the former is more likely. He has a wife and son, and probably a mistress or a girlfriend from his past (I don't know when he dated her because I... show more Dear everyone,

I've been trying to find the name of a movie I watched decades ago. The main character is a construction engineer or architect, but I think the former is more likely. He has a wife and son, and probably a mistress or a girlfriend from his past (I don't know when he dated her because I missed some part from the beginning).

I think she tried to get him for herself, to the point of screwing up his life, so he came to her apartment/hotel room to force her to quit. She stood up on the window and dared him to hit her as he threatened, because if she fell down, people will think that he killed her. He left, and moved to a new apartment.

She followed him to the new place, killed an old lady who had the aparment across his, moved in and let him see her on the balcony. She also made up some excuses to come talk to his wife, you know, the whole psychotic ex-girlfriend deal. Finally, she ambushed his wife and son at their country house/cabin, and planned to burn down the house with them inside, but the boy stabbed her to death.

The female character is pictured as quite strong and a workout buff; she was working out in the scence on the balcony. Also, to depict her murderous side, there is a scene where she went to a bar, flirted with a guy, went home with him and drowned him to death in the bathtub, only by holding him under water with her arms.

I would much appreciate that if you can give me any information about this movie.
Update: Maybe the words "decades ago" throw us off track a little bit. The movie looked like it was made sometime in the late 80's or early 90's; which technically makes it decades old, but it isn't a classic from the 60, 70.
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