A man who kills another demonstrates that he does not accept the principle of individual rights. The proper response is to remove him from society, either by imprisonment or death. Since it is irrevocable, the death penalty is only appropriate when the legal system can establish with certainty that a murderer is guilty. It is arguable whether any current legal system is capable of objectively proving this in every case.
If it were possible to be fully and irrevocably certain, beyond any possibility of error, that a man were guilty, then capital punishment for murder would be appropiate and just. But men are not infallible; juries make mistakes; that is the problem.... It is preferable to sentence ten murderers to life imprisionment, rather than sentence one innocent man to death.-- Nathaniel Branden, "What is the Objectivist stand on capital punishment," The Objectivist Newsletter, p. 3, January 1963.