Is Suicide Always a Sin?

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I believe that in some religions, or perhaps some individuals just believe this, a person will not be held accountable for a sin if they did not understand what they were doing, or could not comprehend that it is a sin. So, if someone was so severely depressed that they could not think rationally, and were no longer capable of identifying sinful acts from righteous ones, would you still think their suicide was a sin? Or perhaps someone with delusions who thought that god wanted him to kill himself, or thought that he would not actually die if he shot himself. If a young child killed himself, or if a person without the mental capability (medical term: mentally retarded or politically correct term: mentally handicapped) committed suicide would you still consider it a sin?