Superman explains to Kong Kenan, the Superman of China, why he doesn’t kill his enemies, even if they have tried to harm his family and close friends, such as Supergirl or Steel. Superman states that if he were to kill them, that would be the pinnacle of their existence, and they would never have the opportunity to atone for their actions or strive for self-improvement.
Because if he dies, Kenan… he can never make it right. If he dies, this is the best he will ever be. Saving him is how we save the world.
From – Action Comics #1053