why did the killer kill??

The guy laid out his reasons. I choose to take them at face value. That and he's crazy.

the directed hinted at it with the rainbow flags. he was a closet homosexual.
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Eric C 4 Prez

Well, there is the possibility that he didn't know she was leaving. She tells Joanne that her parents are picking her up before he walks in to the room. But that's sort of flimsy, especially considering the fact that he kills her parents first (and to followed her upstairs, he must have been...following her...and hence, would have heard her talking to Joanne anyway before she leaves/dies).

But you don't really need much of a reason beyond "he's a murderous psychopath". Because...you know, he is. Slasher films are often interested in motives, particularly revenge and the past coming back to haunt you (figuratively and sometimes literally). But what reason does a serial killer usually have when they slaughter a bunch of innocent people? Not one that is narratively satisfying, that's for sure. It's a base urge. It's all in the brain chemistry, and someone who kills a dorm full of their friends and colleagues has seriously *beep* up brain chemistry.