You talk about the millions of victims as if they would all favour the gallows.
Well, I think we have good reason to think most would. Certainly the opinion of the late members of my family who were holocaust survivors was that there weren't nearly enough Nazis hanged at the end of the war, and that it is an outrage that so many of them only went to jail for a few years or else escaped punishment completely. This, I have good reason to believe due to my talk with many other survivors over the years, is probably the great majority's opinion.
Certainly when Eichmann was tried in Jerusalem, practically all survivors wanted him hanged (as he was), the tiny group of capital punishment opponents (mostly professors from the Hebrew University, for some reason) making lots of noise but gaining little respect.
I certainly don't wish the death penalty on the man who killed my father, for instance. Doing so is -IMO- a position of strength as I have not allowed myself to be dragged down to the same primevil and uncivilised behaviour and blood lust that caused the murder in the first place
I am not saying every single murderer (or killer) should be executed. Indeed, many if not most murders occur in great rage, without much planning, and death would be usually too harsh and in any case not a deterrent.
Murder might be the worst crime, but murderers are not necessarily the worst criminals. (Indeed, most of us, I think, sometimes think, "I could kill so-and-so"; most of us, however, don't think, "h'm -- let's burgle so-and-so's house, he's got nice stuff".)
I do not know the circumstances of your father's death. If he was killed in a botched robbery, or due to a reckless driver, or because a co-worker lost it and shot him at rage over a denied promotion, etc., then I can see why you, as much as you hurt, do not think the execution of the killer would be inappropriate and will be degrading to you.
But we're speaking here of a totally different kind of crime -- of pre-calculated mass murder. If your father was a holocaust victim, one of thousands killed by his killer and his cohorts in the Nazi death camps, if his daily job was to kill Jews, I do not see how you can forgive him in the name of the victims.
You can personally forgive, yes; but, while no punishment would possibly be appropriate for such a crime, a term of years -- say, Speer's 20 years, which, in effect, means a few hours of time per victim (if that) -- is ludicrously disproportionate.