Are the arguments in favour of this sound ? And leaving out deterrence from the equation entirely. Can capital punishment for such crimes ever be proportionate from a retributive justice perspective
Edit: I'm gonna edit this with further arguments if I can
There are various retributive-justice arguments in favor of capital punishment for especially heinous crimes, none of which require that the crime be murder or leave permanent unmanageable suffering. Certain crimes shock the conscience. If the criminal raped and tortured a child, for example, a lot of people will think that execution is the only appropriate punishment regardless if the child survives will eventually recover.
First of all, there is the perceived-justice angle. Will the victim, the victim's family, and others sympathetic to the victim believe that justice has not been done if the perpetrator continues to live? If so, then there is a cost to society in not executing the perpetrator in that it will tend make people cynical about justice. "They care more about the criminal than the victim", etc. Over time, this cynicism about justice may lead to a less law-abiding population.
Second, there is the vengeance angle. If you don't punish the perpetrator sufficient to sooth the anger of the victim's family, it is more likely to create friction between the family of the victim and that of the perpetrator. Someone in the victim's family is likely to try to take vengeance on the perpetrator or someone in the perpetrator's family. Those of us raised in Western societies and not in dangerous neighborhoods often don't see the risk here because we are used to letting the government handle everything, but people who grew up in dangerous neighborhoods or non-Western societies, are far more likely to have experience taking the law into their own hands and be willing to do so. The way we broke that habit in the Western world is in part through the use of capital punishment.
Another consideration is abstract justice. If you believe that good and evil are real things, that there are real obligations and real obligatory justice, then you believe that there is simply a proper response to a heinous crime. The universe has been put out of balance by the crime and to put it back in balance, the perpetrator must lose his life. I'll also note that if you don't believe that good and evil are real, you shouldn't be concerned about capital punishment except for it's functional use. If evil isn't real, then it can't be evil to execute someone.
Very rarely do people receive the death penalty in non despot run countries for non homicide crimes or crimes that don't cause permanent unmanageable suffering. Despots for their part don't need a rational to kill people. If they did they wouldn't be despots.
The US Supreme court ruled in Kennedy v. Louisiana that in crimes committed against individuals the death penalty should not be given in unless the victim’s life was taken.
That said, although it hasn't happened recently (because very few people have been convicted for espionage recently) the US has given the death penalty to spies for hostile countries. The Espionage act of 1917 provides that anyone convicted of transmitting or attempting to transmit to a foreign government "information relating to the national defense" may be imprisoned for life or put to death. IOW someone who caused no actual harm but tried to be a spy can face the death penalty. There were also unsuccessful attempts to broaden the application of this statue later to include among other things people who ratted out the identity of American spies to other countries even though doing so would pose no tangible threat to US security
The rationale for the above is fairly simple. Even though these things may have caused no harm the potential for harm is of such a great magnitude that anyone doing do so is deserving of death. Their behavior risked so many other lives
