Sirius was no stranger to carrying out terrible deeds and laughing at some poor sod's expense. At Hogwarts, he and James ought to have held honourary trophies for the amount of detentions they'd served. And, with the help of the Marauder's Map, all the detentions they had dodged. But serving detention hadn't been too bad. Detention with Filch had been the worst, but detention with a nice bloke like Hagrid or a professor like Slughorn had never taken any skin off his nose. Back then it may have been a rubbish time. But after being forced to watch Regulus and Victoire suffer the way Skeeter had made them suffer... Shining suits of armour and scrubbing crusty cauldrons was nothing compared to Skeeter's idea of repercussions. Sirius had never felt any level of remorse that came near the amount of shame, regret, and rage that he experienced watching Regulus and Victoire endur a boggart without wands for hours and hours on end.
When he was transported back home, he couldn't stay in bed. He couldn't be anywhere near anyone for a while. He needed to be alone. He needed to be Padfoot. Sirius didn't know how to deal with the overwhelming amount of regret weighing on him. He could have drunk himself to an oblivion. He could have smoked himself out of his own mind. He could have gone and burned something else down. But he had enough sense to know that his ways of cooping would have just resulted in more pain and more suffering. If not for himself, than someone who actually mattered to him. And he couldn't take more. He just couldn't.
Not even hiding underneath Padfoot could deafen the ghosts of their screams in his mind.
Eventually Padfoot found himself on Regulus's doorstep and stayed there for some time, whining lowly, before Sirius stood up and knocked. He found himself almost hoping that Regulus wouldn't open the door. He didn't know what to say when he saw his brother standing on the other side. His stomach was twisting in horrible ways. "Hey," he said lamely before something visceral overcame him and he couldn't help but launch himself at his brother, holding him close and burying his face in his smaller shoulder.