It's staged in france nthe girl goes and visits her best-friends family on holiday and the best-friend and family end up murdered,what she doesn't realize that she is the killer

Do you remember a scene where the two girls are fighting and one of them (bad) pulls out a gun and shoots the (good) girl in the back whilst they are in a lake or something? The good girl survies and the other girl is taken away.

Feels like I've been wondering about this film for ages now and I still haven't found it :(

I think what you are looking for is HIGH TENSION.

it's staged in france nthe girl goes and visits her best-friends family on holiday and the best-friend and family end up murdered,what she doesn't realize that she is the killer
High Tension