Why is suicide considered forbidden, and is its perpetrator treated as mentally ill?

A person killing themselves or another is among the gravest of major sins, as Allah Almighty says: "And do not kill yourselves. Indeed, Allah is to you ever Merciful," and His saying regarding the shedding of a Muslim's blood: "And whoever kills a believer intentionally - his recompense is Hell, wherein he will abide eternally." And as the Prophet, peace and blessings be upon him, said: "The perishing of the world is less significant to Allah than the killing of a Muslim man." The reason for the prohibition is the taking of a human life. The sin is established if the killer is sane and aware; otherwise, the pen is lifted from them, like the insane person or the child. Not everyone who commits murder is mentally ill.