While, for the most part, I agree with you, there are cases that are simply a textbook example of needing the death penalty. If somebody, in their right mind, decides to kill simply because they want to know what it’s like, they need to be removed from the herd.
Look at inmates who continue to present a danger not only to staff, but to other inmates. If, as far as medical science is able to, they are in their right mind, what do you do with them?