One of those plots which have become familiar to the point of boredom when it comes to noir thrillers: psychopath out for revenge, and going a convoluted way about it. Throw in characters who are supposed to be reasonably intelligent, but who perform the dimmest acts imaginable (e.g. Vincent Mandel, the main protagonist, has just been falsely accused of rape, but he agrees to meet the girl again - ALONE, AND WITHOUT TELLING ANYONE!!); and a tedious domestic crisis, which partly accounts for Mandel's decision making (but doesn't really); and we have nothing but a shell of a film.
The actors give typical performances for something of this kind: Yvan Attal has fraught bemusement on his face for most of the duration. Clovis Cornillac is the hard man with malign eyes, who we keep seeing brooding malevolently.
Thrillers depend upon a suspension of disbelief for the length of time you're watching. 'Le Serpent' does not provide that for a moment. Watch it to see what happens, if you can be bothered, but don't press too hard on any point of interest other than that, because the whole thing will break like an Easter egg.