What Sirius didn't know was that Voldemort, had he known what Regulus was going to tell McGonagall, would tear up the city to destroy him. He wasn't your average traitor. And Regulus was already hated for being a death eater, that's all he needed was to be the reason why these people's lives were ruined even further.
Regulus grabbed the railing and walked upstairs, it hurt his thighs, his travels around the city and such were harder on him than he expected. But he didn't let his pain show, not in front of him. Didn't need to be mocked further.