What part of the world could they not keep people in similar conditions as a supermax? If it’s a matter of money and that’s the only reason they don’t have them, that seems like an argument in favor of funding them, not in favor of the death penalty. I can’t think of another reason.
Incidentally, the fact that the U.S. is the only Western country with the death penalty, sharing that honor mostly with theocracies and dictatorships, should tell you something about the ethics of it.