Or it's a form of societal self-defence: there are some folks that will continue to be a menace to society, whether they'll do murder or other bad things, and society wants to eliminate the risk/threat.
If someone comes at you with a knife or gun, you have a right to protect yourself, potentially up to the point of killing the attacker. If someone comes at a group of people (e.g., at a temple, mosque, church), they have a right to protect yourself, potentially up to the point of killing the attacker.
If someone keeps coming at member of society, society has a right to protect itself.
In modern times, with modern prisons, it is much easier to keep these people isolated from society at large, and so the need for the mechanism has been diminished, but the principle is still there.