Severus is willing to roll with that assumption - with anyone else, it would be true, but concerning Bellatrix, he hasn't asked because of the likelihood of what the answers would be. Bellatrix Black may not yet be Bellatrix Lestrange, but the latter did not spring fully formed and psychotic from Zeus's head. He doesn't really want to hear about what she likes doing. He suspects, however, she's not going to tell Ilde she likes torturing Muggles, and so his silence and continued consumption of his cupcake goes on unbroken.