"Why don't you ask him?" Rodolphus says, and it is a sort of joke, as he had done earlier, but not one nearly as good-natured. It could even be seen as accusatory, as if Antonin were somehow supposed to know (or care) that Severus had killed Bellatrix before he started hanging out with him. As if Rodolphus knows and has evidence this is true, as opposed to something he only believes because of things Narcissa did and did not say.

With effort, he shuts his eyes, containing the anger, the uncharacteristic sharpness, and putting it away wherever such things go to fester and ferment.

"Don't ask him. Why shouldn't he? It was not the Bellatrix here now. That one was older." With all the ensuing implications.