The state using the legal process being trusted to do something is different between an individual doing so. The state executing people is the only way that justice can be achieved; there is a moral difference between execution in support of society and murder against society.  There is an immense difference between a murder and a lawful killing by the state. If the death penalty makes the state no better than a murderer then a soldier is one too. In a more absolutist view, if capital punishment devalues life, do fines for theft devalue property?