However, some have argued that the case for execution isn't primarily a matter of deterrence or even expense, it is a matter of retributive justice. I do accept C. S. Lewis's claim that it is extremely perilous to remove the question of desert from sentencing.
It should be the first consideration, though surely not the only consideration. I am quite sure that Lewis would have also endorsed this comment, which his friend J. R. R. Tolkien put into the mouth of Gandalf:
“Deserves it! I daresay he does. Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgement. For even the very wise cannot see all ends.”
The retributive theory of punishment requires that we deprive the criminal of happiness to a degree commensurate to the wrongness of their acts. In order to fit the crime, the punishment does not need to resemble the crime. We wouldn't use that principle in the case of rape and torture, so why use it for murder? In order for the argument to go through that the death penalty uniquely meets the requirement of giving a criminal his just deserts, you need an argument other than the argument from resemblance, and I don't know what that would be. Executions are quick and physically painless, which was probably not true of the death of the victim of murder. The person executed knows for a long time that this is coming, which again would not be true of the victim. So, once we are deprived of the argument that a punishment that resembles the crime best fits the crime, how do we show that the death penalty is the best way of exacting retribution?