BEWARE: These comments give away crucial elements of the plot!!! Don't read these comments unless you've seen the movie!!!
Even though I don't find the movie works well as a thriller, I am glad I watched it. Here is why:
Assume that your behavior is determined by your nature, i.e. by the genes that have been passed down to you from your parents. Is it then still possible to hold someone responsible for what he or she is doing? In other words: Why do things happen the way they happen? This is IMHO the fundamental question that Claude Chabrol asks in his latest movie Sweet Poison.
From very early on in the movie the alignment of characters is fairly obvious: A couple consisting of a femme fatale and a detached pianist, their dull son and as a twin personality the young, alert, and beautiful woman, and her mother, a doctor. Whereas the social relations between these characters are plain: couple, son, daughter, the biological relations between them are highly questionable: the son had been conceived by a woman who later on died in a car-accident; daughter and son might have been swapped on their very first day of life; the doctor conceived her child with the help of an anonymous donator of semen; the femme fatale had been adopted by her parents. This absurd number of ambiguities seems to indicate that this is really the main theme of the movie. The viewer is led to believe that the swapping actually took place and that the daughter has inherited the musical talent from the pianist, while the son inherits the dull unspecificity of his anonymous father.
All four main characters - the couple, son, and daughter - simply live out what has been given to them by nature: the father is a famous pianist, his daughter follows his foot-steps. The femme fatale (symbolically portrayed as a spider) tries to kill the women that get in between herself and the pianist (the mother of the son and the pianist's daughter). The daughter lives an interesting life, which includes playing piano. The son doesn't act at all.
The femme fatale kills the mother of the pianist's son with the help of sweet poison (reflecting the German title: Suesses Gift). When the daughter starts to interfere with the life of the couple, the femme fatale takes the exact same steps (not buying drugs, hurting her son's foot, sending the woman into town to buy drugs, mixing sleeping drugs into the woman's drink) in order to kill the daughter, too. She behaves like a spider that builds a web and immediately starts to build another one when a scientist destroys the web the spider just made: It is a built-in program that's running, not something that the wasp decides to do or not to do. When the pianist finds out about his wife's nature, he doesn't grab her by the throat or accuses her. He just asks her why she did it and then goes on to play piano. This answers the first question: If humans are driven by their nature one cannot hold them responsible for their deeds anymore. Because it is just their nature and they cannot help it.
However, when the femme fatale tries to kill a woman who is close to the pianist the second time around, she fails. Her plan goes the same way as the first time. Whether she succeeds or fails depends on chance, i.e. circumstances that lie beyond her influence, like better car technology. What determines the outcome of things then - the second question - is not human will or drive, but random chance. It is nothing but luck whether things work out or not, if we assume that it is all in our nature/genes.
