They were both beyond words at that point, too many years of anger and hurt and finally a place to put it all, even though Sirius knew this wasn't who he wanted on the other end of his wand most. Snape was someone from his past, and easy target when they'd been at school and in many ways the cause of all his pain, but he wasn't the one who had betrayed them. And he wasn't the one who had sold out his best friends. But he'd have to do for now.
Sirius stumbled, reaching for his side like that would be enough to ease the pain he felt though so much more of his actions were fueled by that anger and pain and he flicked his wand again, wincing as the motion pulled at something, not just the invisible bonds that kept them close but something else and he clutched at whatever piece of furniture was nearer, trying to keep his footing but he could feel his vision tunneling, the world going dark around the edges as his heart raced, pumping blood through his veins and spilling out over his clothing.
"Do it," He hissed, at least then he didn't have to face Snape any longer, he didn't have to try and be the better man or anything at all.