You're not quite sure what drove him to do this. You thought the two of you were good friends. Sure, you two fought sometimes. Mostly it was banter and the other times... well, you just never thought he would do this.
But honestly, how could you be surprised with how he had been acting recently? You knew he stopped taking his meds and you knew he was getting worse, but... hitting you? Going that far? To someone he openly stated was his 'best friend' one night when the two of you got piss drunk in his dorm room? You can't really blame yourself for being a little caught off guard.
He just stood there. God, you just realised how terrifying he could be when someone sets him off. He didn't even look sorry at the fact that he had just back handed you across the cheek. And he did it hard. Like he had been waiting to hurt you for way too long.