I think the thing that makes that scene so problematic is the question isn't so much who's to blame, but who's to blame for what? Draco starts the fight and escalates the fight over Harry innocently (basically, even if he was trying to catch Draco there there's no crime in walking into the room) entering the bathroom. He's responsible to the fight. But then there's the question of who's to blame for Draco ending up bleeding out on the floor, and I think then people switch to Harry choosing to use a spell that did that, and what he could have expected, etc.