What is the difference in evil and mental illness?

Say someone killed someone, how do you know it was mental illness or just evil?

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If he isn't sane anymore we treat the person like a baby. Like: oh he didn't know what he did. I don't know exactly. This is a bad comparison... But then the person can't think clearly.

But if someone did it while being sane then the person knew it. Knew what he or she did in the moment and then the person did it in purpose.