I'm a lightweight, but I'm going to put this on the list just because the violence in it comes out of freaking nowhere:
A Ma Soeur!
The title translates to "To My Sister", but it was released in English as "Fat Girl".
It's about two sisters: an older, pretty one and a younger, fat one. For most of the movie it's about the jealously that comes up as the older one has her first sexual experience (while her sister is in the room - apparently they thought she was asleep) and the fat girl deals with her poor self-esteem and jealously of her sister's sexuality. There's a lot of full nudity, even from underage characters. Most of the time it isn't gross, just frank. It's mostly intellectualizing about female sexuality at a young age.
Anyway, eventually the parents find out that the older daughter had underage sex, and there's a long car ride with the two sisters and their mom. Pretty girl is embarrassed and depressed, the mom is mad, and fat girl just wants everything to be over with. So after a boring road trip, they stop at a gas station and fall asleep at the car.
Then, just out of absolutely nowhere, with nothing setting it up, a crazy guy smashes the windshield of the car they're sleeping in, busting pretty girl's head in and killing her. He strangles the mother and rapes fat girl. The next day police are sweeping the scene and fat girl denies that she was raped.
So, the movie ends by killing two characters, and raping the main one,
for no reason at all.