With all of Bellatrix's strength going into the curse, it hit Sirius hard, momentarily driving back his own voice as his mind went blissfully blank, filling with her instructions. He was a purist, and this half breed deserved whatever it got. He sat calmly, even with blood covering his hands and a rather spectacular splatter of it on his robes, watching with detached interest as Bellatrix bound the beast. He was, no longer, in any mood to fight or refuse or even really think; his mind was tired, and it was almost a relief, really, only having one voice to listen to.
"Ta-ta," he murmured as Bellatrix put her boot on the small of Remus' back, face expressionless as the dark creature got what he deserved.