As much as Bellatrix tended to adhere to certain rules of propriety (the ones that suited her, anyway) there were always exceptions. And this was one of those moments as she could not resist the opportunity to exploit both Remus's feelings for Sirius and Sirius's obvious enjoyment of their little torture session. Not that she could not understand Sirius's reaction - there was something incredibly arousing about seeing another person suffer, at least as far as Bellatrix was concerned, she just did not reveal those feelings until she was home with her husband.
"I did not tell him to enjoy this," she whispered in Remus's ear as she dug her nails into one of the deeper cuts on his stomach and used her free hand to push Remus back into the chair. "Only to make you suffer. That smile, the way he is looking at you, that is all him. This is who he is when you strip away all your ridiculous Gryffindor nobility." She gave another twist of her nails inside of his wound before looking down at Sirius and smiling. "Make him scream even louder. Make him cry your name in agony. And then tell him how it makes you feel."