Sirius Black was mourning.
He was in his house - the house he was using as a safe house - and was sitting in his room, curled up in a ball. Sobs wracked his body as he relived the moment when he found Lily and James dead, the moment when he found out that Harry was alive, that he, Sirius, could look after him . . . But Hagrid, dear old Hagrid, had said no, no, Dumbledore was going to find somewhere for Harry, that he wasn't supposed to let anything stop him getting to Dumbledore with Harry. Sirius had knew by then that he wouldn't be able to talk Hagrid into giving Harry to him. So he gave his bike - his bike that could fly, the one that James had helped him buy - to Hagrid, stating that he wouldn't need it anymore. Hagrid hadn't known why but he was thankful for the faster method of transportation. But obviously Sirius knew. He needed revenge, revenge against Peter, the traitor, the rat. The one who told Voldemort where Lily and James were.
Fury welled up inside of Sirius as he thought of the thing - for he did not deserve to be called a man - killing James, then Lily, and trying to kill Harry. But for some reason the monster hadn't been able to. Sirius didn't know why but Voldemort hadn't been there when he had gone to see, to check, that maybe the Potter's, his best friends, were all right and he was wrong and Peter hadn't betrayed them - but the point remained that he had betrayed them and nothing was going to change that.
And Sirius knew it was his fault. If he hadn't convinced Lily and James that they should try making Peter Secret-Keeper - "He wouldn't do anything," he had said to James while Lily had tried to sooth little Harry into a slumber. "He's just Peter." And Peter had betrayed them. Betrayed them all . . . Sirius knew what he needed to do. He needed to avenge Lily and James' deaths. He wouldn't go after Voldemort - he was not suicidal. He would go after Peter, the traitor, and make him pay. He would.
He would.
But he didn't.
Ugh, emo moment for me when I wrote this. I was feeling crappy at fencing, losing and stuff, thinking of how annoyed I am at everyone for being on Pottermore all the time so I can't because of 'overwhelming deman' ... and I wrote this is half an hour.