Very interesting omission indeed!! Accidental? Who knows. But it does change the feel of the conversation quite a bit. I took it when I read that part that Dumbledore was referring to the fact that a person loses part of their soul when they kill someone. Thus, "He cannot kill you if you are already dead." You've already sacrificed part of yourself by doing the act. Being a DE and killing is as good as being dead you might say. Your soul will never be complete again.

Now quite a different theory may refer to Dumbledore's death. Could this mean something there? He cannot kill you if you are already dead... Are we going to theorize that Dumbledore was hiding something? Perhaps Snape killing him was not what it appeared to be (which I felt anyway)? DD already seemed to be at the point of death due to the potion he drank. But I'm not sure that there were hints that he was already dead at that point. The painting of him appeared after, when MacGonagall took his office. Must read more...