I've already said it but I'll say it again: NOTHING excuses his behavior. I understand why, but that doesn't excuse it. You didn't see Harry turning into a bully because of the Dursleys. People CAN overcome those things. Snape DIDN't. Yes, his life was sad and pathetic but nothing, I repeat. NOTHING excuses how he treated Harry (even when he supposedly 'cared' for Harry) or ANY of the other students!