Cromwell didn't murder tens of millions of people. The Nazis did. But no, I don't support executing dead Nazis, only living ones.
Napoleon may have caused the death of as great a proportion
of Europe's population as did Hitler, yet the British chose
to exile him (twice!). I think they were correct in doing so.
The Kaiser was exiled. I remember seeing him in a newsreel
in the thirties chopping wood in Holland. I think that it
was a great mistake to try the leaders of the defeated powers
after WWII. After all the Russians, who killed more people
that the Gemans, got offf scot free.