The point is that if you are certifiably mentally ill, you are not in your right mind. Severely mentally ill people are in a mental state where they cannot understand whether what they are doing is either right and wrong. Its not that their actions (murder in this case) are justifiable, its that they are culpable for them. This is why I always cringe when people say things like "these terrorist bombers are mad". No they are not mad. That is the frightening thing. If they were mad, I could understand and even excuse their actions. But they arent. They are completely lucid. Not mad but evil.
Of course, what the shrinks never talk about is why incidences of "paranoid schizophrenia", "multiple personality disorders" and so on are so rare in countries where having them is not a legal defence against murder :) IOW, people are playing the system...claiming insanity when they arent in order to avoid punishment.