It's horrible. He knows it's infinitely worse for Sirius but just watching it regularly makes him ill, much less watching Bellatrix do it to his brother--it goes on so long. When Sirius swears at Bellatrix though he knows it's a mistake - his eyes widen slightly, and then Bellatrix is speaking to him. He startles and doesn't answer, but she takes it for speechless excitement and smiles, drawing him up closer.

"Go ahead, Regulus darling. I know you've been itching for this, I can see it in your eyes."

What's in his eyes is terror, but she doesn't see anything other than what she wants. He exhales shakily and raises his arm. He knows the charm will hold but his hands are shaking so badly that she's going to notice and--

"I can't."

There's a sickening silence.

"What?" It's sharp before she turns sweet again, shaking her head like he's said something funny. It's still dangerous. "Of course you can, Regulus! I've always known we've needed to toughen you up and this is the perfect chance! Here, I'll help you."

Her 'help' is to hold Regulus' wand arm with hers, guiding him to point it properly at Sirius and lean close to his ear to speak. "Just think, Regulus. Just hate. He abandoned his status, abandoned his family. He deserves this, doesn't he? We've kindly given him a chance to come home and he's thrown it in our faces."

Regulus' grip on his wand tightens and he exhales sharply through his nose. She's steadying his arm still but his hands won't stop shaking--finally he croaks out the tiniest whisper, barely audible.

"I can't."