Severus wasn't willing to dignify that response with more than a sneer. He doubted that she even saw that with how quickly she had turned around. With a roll of his eyes he started for the kitchen, thinking he could use tea as an excuse to leave her. He only got two feet before she was going off on him about how it was all his fault Sirius Black was dead.
She was in mourning, and he accepted that. And he knew that with mourning often came bitterness and anger towards others, and he could accept that, too. But he couldn't accept being blamed for deaths that he didn't cause.
"Excuse me? Are you implying that I am at fault for your cousin's untimely death? I'm sorry to tell you, but if he hadn't stupidly left then, he would have stupidly left some other time. My maturity level- although much higher than his was- has nothing to do with it. He was bound to escape eventually. Do not blame me when his lack of intelligence and Bellatrix are the only things that can be held responsible for his murder."
Severus was a lot of things, but he was not immature, that was for damn sure. And he certainly wasn't on the same level of maturity as Sirius Black was. Perhaps she wasn't in mourning, perhaps she was in denial, if she was thinking such things.
She was turned from him and so he couldn't confirm if her sniffing was out of anger or because she was crying, but he did see that she had begun to drink. If he knew her better he'd stop her, only on the principle that it was bad to drink alone, which he'd learned from experience. But he didn't know her well enough, so he restrained himself.