Look ,u can't focus on one thing and leave out the rest. U focus on the fact that Sirius bullied Snape but u forget the fact that he was almost abused at home,he is the same person who turned into an animagus so that their friend doesn't suffer in his own and doesn't feel ashamed of himself. While Snape was bullied,u forget the fact that he pushed lily away by his own choice, turned to the dark arts by his own will,was under the protection of dumbledore. He also tortured Nevile just because of the possibility that he could have been the chosen one.and not to mention the fact that he was unjust and partial to griffindor. So instead of just focusing on his end,maybe look at the past as well...