Disclaimer: I don't own any of the material for the Harry Potter books; I just like to mess with the character's heads.
Murder. He never thought that word would be associated to him. He knew it was wrong while he was doing it especially after words when Snape was shaking like an epileptic in the throes of a seizure and paler than any live human he had ever seen.
He knew it was wrong but it was just Snivellus. It wasn't like he actually mattered to anyone. Hell even his parents hadn't shown up after Dumbledore had sent them a patronus. He obviously didn't matter to the headmaster who had punished Snape for almost getting murdered.
There was that word again. Was it really murder if you didn't see them as human. Sirius didn't think anyone could consider Snape a human being. So he had gotten off with a slap on the wrist while Snape had gotten in trouble for falling in to a trap. The lovely little ironies of life.
Sirius had seen him around. Snape was a paranoid little bastard and almost getting killed just made it worse. The fact that he actually let a near death experience change him seemed to anger Sirius. How dare Snape act like an actual human being how dare he.
So when Snape walked by Sirius insulted him, humiliated him, and hexed him with more regularity and hate than ever before. Snape didn't get to act like he was bothered he had to act like the world couldn't touch him as he had before.
That's what made Sirius hate him in the first place. That nothing could touch Snape, nothing could hurt him. Sirius had never been able to do that. He felt hurt when his mother disowned him, his brother turned away from him, and his father watched as if he were a casual bystander. Snape didn't feel hurt or jealousy, he didn't get scared, except he did.
If Sirius had hated him for the fact that he didn't feel he hated him even more now that he did. Now Snape was just angry at the world for dealing him a shit hand in life, the same as Sirius. Snape should never ever be similar at all to Sirius because Sirius had tried so long to prove that he was better than Snape and he would never be anything like him.
The beginning of next year when Snape showed up for school with fading bruises on his skin, the after effects of the Cruciatus curse, and the dark mark branded in his left forearm Sirius couldn't help but hate him. He had lost the battle that Sirius had fought for so long.
Snape was everything his mother would have been proud of but Sirius couldn't be.
He was twenty one. His best friends just died because of a traitor they thought to be their friend. Sirius of course went to confront him. Peter always had been the weakest of the bunch just happy to float after them never having any original thought of his own.
Sirius had never felt so betrayed in his entire life and if James and Lily were here to tell the tale he was sure they would feel the same way. So he went to confront Peter with murder on his mind. He got only as far as cutting off one of Peter's toes when chaos ensued and Peter got away. Next thing he knew there were twelve dead muggles and no one to blame except him. Once again he was labeled a murderer except this time there was no slap on the wrist he was hauled off and thrown in a cell in Azkaban.
He had never been so cold in is life he felt it all the way down to the marrow in his bones. Every bad thing he had ever done was coming back to haunt him from the time he had broken his mother's favorite vase and blamed Regulus to when he had tried to kill Snape. When he actually tried to kill someone he was totally blameless and now that he was innocent he was lying freezing in a jail cell. Seems irony had come back to bite him in the ass yet again.
He was a murderer they all said and there was no one to believe otherwise.
Years later as he is sitting in his dreary kitchen and listening to Snape give the latest report on Deatheater activity Sirius can't help but hate the man even more. He had done everything Sirius wished he could do. Snape protected Harry, got to play an active part in the war, and could actually leave his house.
Then he hears that word again. Murder. This time it isn't attached to him but Snape. Snape actually was a murderer not just an attempted one. He had killed and Sirius didn't envy him because he knew what it was like to nearly take someone's life and then try to justify to yourself that it's okay.
That word would never ever be attached to him ever again because for the first time in his life Sirius Black didn't carry an ounce of jealousy as he watched Snape, who was close to collapsing, report atrocities that Sirius knew he couldn't bear to witness.
He finally conceded that Snape although an ugly git (and many other insulting words but Sirius was saving those for later) was probably the bravest man he would ever know.