His work with the Order of the Phoenix was brave, noble and worthy. But in no way did it make up for his lifetime of bad deeds. Before he became "good", he was a pure-blood supremacist, killing and torturing people on Voldemort's behalf. He ruined his relationship with Lily through his own bigotry, and spent the rest of his life taking out his pain on everyone around him. Yes, he was bullied at school. But this is no excuse for the way he turned out.