I caught this movie very late at night on HBO back in the early 90's. All I can remember is the plot:
It starts off with two friends who play lacrosse together. A girl moves next door to one of them (the good guy), but I cant remember if he already knew her or not. Something tells me that she was there for the summer or something. But anyway, they go out and the bad friend gets her on drugs. There is a scene where they are at a club and he gets her to snort cocaine in a bathroom stall. They then have sex in there and the good guy sees it.
The movie kind of remains that way, and the good guy is not happy about the relationship at all. It climaxes at a seen at a party the girl is having. The neighbor girl's younger sister has a crush on the bad guy. He ends up raping her, but does not tell anyone. It then ends at a lake house, where the good guy ends up getting the girl, and then kills the bad guy in a row boat out on the lake. I specifically remember the bad guy being on acid or something at the time.
The bad guy either is or looked a lot like Rob Lowe, but I cant find anything under his name in IMDB. Some people have said Less Than Zero, but it is not that movie. This was definitely a drama movie.