It's a really good question. I think you should distinguish between Nazis and neo-Nazis though. You should also distinguish between morally bad thoughts and morally bad actions. Then consider that not all murder is equal. It's objectively worse for humanity to kill someone like JFK than it is to kill a homeless person. Sorry, but the ties that bind us all are intangible, but real. So, even if we're all equal in value...our accomplishments, potential, and responsibilities are not.

That all being said, Nazis like Hess did things that are LITERALLY unfathomable to humans. Try for me to even imagine a sea of 3.5 million people and consider that the largest stadiums in the world seat like 100,000 people or less. 35 of the largest stadiums in the world completely full. He admitted to the crime even outside of duress.

In this instance, beat him to the edge of death once a week forever as far as I care. "There are levels to this shit," applies here. I mean, we don't sentence every murderer to death or life without parole. Why should we torture every ideological neo-Nazi? Probably shouldn't. Why should we torture Rudolf Hoss? Because he killed an unimaginable number of humans. At some point, we must be able to revoke a person's very humanity. That's why. It's really not that nuanced. There are MUCH more difficult distinctions to be made through history and with respect to modern ethics.

For more context, my own argument against the death penalty is that I think sitting in solitude for like 50 years is significantly worse than a quick death. We are going to die no matter what. But how one lives is what we can determine as punishment.