It is American society, through it’s elected government, which determines which crimes are punishable by death. In fact, it is American society, through it’s elected government, which determines all penalties for all crimes.
There are lines and gradations drawn all through the American system of justice.
If a policeman stops you for speeding, you pay your debt to society (for making the society’s transportation systems just a little less safe) by paying a monetary fine (unless you are habitually stopped for traffic offenses, in which case your privilege to drive may be suspended or revoked).
If you get caught robbing a bank, the penalties are higher. In this case, you will likely be imprisoned (lose a large measure of your personal freedom) for some extended period of time. Even if persons are killed in the robbery attempt (as long as it is able to be reasonably demonstrated that robbery, not murder, was the primary objective), it is likely that you will lose your freedom for a very long time, but keep your life.
However, American society, through democratic means, has sanctioned the death penalty for what it considers to be the most heinous of crimes. Therefore, if a person, of otherwise sound mind, with malice and aforethought, deliberately and premeditatively sets out to AND succeeds in killing members of American society, that society has concluded (for this time) that the death penalty will be an option to be considered.
If you agree that there is no objective standard on which we base our societal laws, ethics, and values, then the establishment of such penalties is left up to the subjective determinations of the individual societies in which we all live. There, effectively, is no right or wrong, other than that which each society dictates. If you disagree with the determinations of the society in which you live, you have the opportunity to politic to change those aspects of society with which you disagree, or, if need be, relocate to another society.
JustAThought