The only person using rhetoric here is you. There are morally depraved people out there whom we colloquially refer to as “evil.” I don’t know why you insist on having a semantic argument. If “[moral depravity] does not exist,” as my interlocutor claims, then nihilism would indeed be true.

I would also like to point out that the ethical arguments against the death penalty in the scholarly literature are very weak and it remains an open question whether the death penalty is advisable on practical grounds. Morally it’s unlikely that any good argument exists to make it impermissible to kill “evil” people. You can check out the latest edition of any textbook on ethics, such as Living Ethics by Schaffer Landau, which syllogizes a variety of arguments on this topic.