That principally nobody deserves death is not universally valid within the nature, within the animal world. There are no rights but just struggle for survival 24 hours a day. Eat or be eaten. It would be naive to think that animals would not murder and not conduct war if they would be equipped with capabilites our species is equipped with, genetically and technologically, just by the way. Remember where we came from.
That nobody deserves death is a decision. A human decision, based on rationality. Based on the realization that we don't need to murder in order to survive. Based on the ability to realize that I don't want to do somebody a mischief because I don't want that somebody does a mischief to me. A brutally cold murderer decided for himself that his victim deserves death. No moral, no god or anything else did stop him or make him think, decide and act otherwise. It was his decision. He decided that he has the right to kill somebody. And he should accept and expect that others also take decisions. The decision might be custodial sentence or it might be capital punishment. The difference between the decision of lifetime custodial sentence and capital punishment is small. But the difference between coexisting or not coexisting on that planet with a murderer of loved ones is huge. A murderer has taken decisions. And it's up to us to take decesions on him. There is no superior universally valid instance or moral that decides anything. It's humans that decide.
I remember an interview with a husband of one of the teachers that got killed by German spree killer Robert S. (the Erfurt massacre in 2002). He said, distinctly and calmly, that he is "glad" that Robert S. has killed himself. Otherwise he would have done his utmost that Robert S. would not live anymore, which is not compatible with his believe as a Christ (funny though...). I can understand that husband. I think (and would act) the same way. Family is a sanctuary. If somebody decides to destroy my family, I would take decisions as well. And those decisions, in case of murder, would be based on my unability to coesixt with a murderer of my loved ones on this planet.