Sirius would be lying through his teeth if he tried to claim this wasn't arousing in a fucked up sort of way, with Bellatrix curled around him, Remus screaming and pleading and bound in front of them, and the absurdly sexual sensation of actually piercing Remus' flesh with the knife. His eyes were very dark as he gazed at Remus, looking almost thoughtful as he watched the trace of the blade across his throat, guided by Bellatrix's hand. He listened intently as his eyes followed the knife, filing away where he could push deeper, where he had to be careful, what got the most reaction out of Remus.
"Oh, but I do," he murmured, "so very much. I believe we both do, so you lose this battle, Remus." He began to guide the knife himself now, knowing Bellatrix would catch him if he went to hit the wrong spot, and he began mirroring the path that they'd made over Remus' throat, going over what he had begun to memorise.