Of course the death sentence is evil. In fact all punishment is evil.
It is with this evil that the law commands your obedience. The law is an authority and its authority sits in its ability to oblige or put you under an obligation to perform or refrain from doing something.
There is no other way. If the law cannot do this, it fails to be a law.
Lady Justice holds in her right hand a sword, not a cane. Justice is not there to give you a smack on the wrist and trust that you would not be a bad boy again. No, once convicted of a crime she delivers you a pain and that is the end of the matter. Iudicium finitum.
If you commit an offence, the law will inflict an evil upon you. The greater the seriousness of your offence, the greater the magnitude of the evil. Death is the ultimate evil that justice can inflict on you.
One might even criticise that it is hypocritical to profess to uphold rights to freedom, life, liberty and property and then selectively chose to make a punishment of imprisonment and fines whilst doing away with the death sentence. Or to create facilities extra-terrestrially and then do all these things (including torture) whilst espousing human rights.
You may say that it is wrong to put someone to death because it is irrevocable. But it is equally wrong to deprive someone of freedom (for any period of time), to inflict pain or to seize money and property from another – all these cost permanent loss. Some of this physical and mental anguish can be so devastating, convicts have been known to prefer death instead.
Deterrence, retribution and incapcitation are just some of the functions of punishment – it can be, but at the same time it doesn’t have to be. It is not about “an eye for an eye”, the law could have no care about your eye or if anyone goes blind. It is not concerned with your opinion about fairness – the concept of fairness is empty and subjective.
Punishment, as far as justice is concerned, has to be proportionate to the criminal neck it seeks to grip. If it were not proportionate, it would do great injustice to the rest of the citizenry.
And the most unjust of all; punishment if not proportionate would do great insult to the victim.