That cackle sends a chill down his spine and it's stupid but he flicks the lights on anyways, because at least if it's her he can fight her better if he sees her. He's quick about it, already spinning back to face her before she speaks. There's a snarl on his face as he aims his wand in her direction, one that only grows at her 'question.'

"Like hell am I going to join you and your band of nutters, Bella," he growls back, wand moving before he's even decided what spell to use--

--But then two things catch up to him at once: That Bellatrix had said 'dear cousin,' and hadn't been talking to him, and that there is Regulus, standing behind her with a look of fear briefly on his face that feels too familiar. It cuts Sirius off mid-word.

Another mistake in a line of many, really. Responding with this much shock leaves him wide open and vulnerable to any attack Bellatrix chooses to fling his way.

(The part of him that recognizes this is numb, quiet. Nothing matters when Regulus is afraid of him and Regulus is here and this is all Sirius' fault, for leaving him. He deserves whatever's coming to him at his brother and cousin's hand, for abandoning him there and making him into this.)