Suppose Hitler didn’t kill himself and was captured alive by the Russians in Berlin, or by Israeli commandos in South America. What would we be morally allowed to do to him if we had managed to capture him? Does a person like him have human rights that we have to respect? Of course. Whatever dehumanizing name you wish to call him, he was a human being like the rest of us, and we have to deal with that fact. Every human being has rights and those rights are not conditional upon good behavior. No one has less or more rights than the next person. It’s not because someone has committed horrible crimes that we are allowed to take away his or her rights.
Hitler’s rights include a right to life. This right is quasi-absolute and can only be limited if that’s the only way to save other lives. So for instance, we are allowed to shoot him on the spot if he resists arrest and threatens to kill us or others (such as hostages). But suppose Hitler is captured alive and is no longer a threat to the lives of others. Shooting him is then not allowed because that would be an extrajudicial execution.
Are we allowed to execute him after a proper trial? Maybe a living Hitler who’s kept in prison would still be able to encourage his followers to continue their murderous rampage and maybe that’s a sound argument for executing rather than imprisoning him. But I think that’s a far-fetched scenario. Only in the unlikely case that there is a real risk of an imprisoned Hitler ordering murder and that executing him is the only means to remove a threat to the lives of others, would his execution be allowed. This is equivalent to the case in which Hitler is holding hostages. However, even in this case, going after Hitler’s followers would be more effective.
So capital punishment is not an option. Remember also that other justifications of capital punishment aren’t available: we are not allowed to deter future criminals by killing present criminals, not even if it works, since that would be an instrumentalization of a human being. Going down that road ultimately leads to the devaluation of all human life. Life imprisonment without parole then? Not an option either because even Hitler can be rehabilitated. The problem with rehabilitation is that you never know who can do it until they do it. You can’t say in advance that some people are beyond rehabilitation.
Some form of criminal punishment is obviously warranted since Hitler acted with intent, knew the consequences of his actions, caused the consequences of his actions, wasn’t forced to act, was aware of alternative courses of action, violated existing law and was found guilty of such violations after a fair trial (ex hypothesi). Given the unavailability of capital punishment and life without parole, some fixed term prison sentence seems to be the only remaining option. And I know that’s a huge anticlimax for most of us.
But what do we want to achieve with that sentence? Retribution? Even if retribution is a justified end of punishment – which it isn’t since we should in general try to be better than criminals - a fixed term sentence is hardly retribution for Hitler: on any account, this is less than what he deserves. And more than this is ruled out (see above). Not only aren’t we morally allowed to execute him, but even executing him doesn’t seem enough. If anything, he deserves to be executed millions of times over, which we obviously can’t do even if we were morally allowed to do it.
Perhaps we want to achieve incapacitation. That’s reasonable enough in this case. You can hardly allow Hitler to walk the streets. But again, this is truly anti-climactic. It leaves us with our anger and sadness. But I guess there’s no way to leave our anger and sadness behind in this case. The morale of this story is that the same is true in many other, less extreme cases as well. We tend to be too ambitious when punishing criminals.