Severus clenched his jaw and didn't answer her, because he already knew in his heart that if it had been anyone else, any other family, any other baby, he wouldn't have cared. At least, not enough to do anything about it. There had been few people over the course of his life--perhaps 10 at most--that Severus had ever truly wished violence on, but for everyone else...no one but Lily had ever given enough of a damn about him to warrant caring about what happened to them. Did he want people to die? No, he didn't. But neither did he go out of his way to intervene and stop bad things from happening to others.
"I know that, that's why I tried to save you, I'm the one who told Dumbledore you were in danger, who insisted you go into hiding and use a Secret Keeper!" It was all he could do to keep from rising to her energy level; keeping his cool when confronted with rising emotion had always been one of his struggles, and he could count on one hand the number of times he'd successfully not risen to the bait--that was one of the reasons he'd tried so hard to never fail the Dark Lord, because failing to control his emotions in front of him would have cost him his life.