Well, on a matter of killing Hitler, or just letting him die, a very black humour story from where come from pops in my mind...
A man goes to see a psychic, and she tells him, even though he is a decent and honest person, he will be responsible for the deaths of millions of people. The man was desperate, so to soothe his nerves, he took a walk by the river. As he was walking he saw a boy drowning, so he thought: "If I am really going to be the reason for millions of deaths, let me save just this one life". He jums into the river, saves the boy. The boys mother comes running and hugging the boy, then scolding him and so on. The man starts feeling a bit better about himself. The woman then turns to him and says to her boy: "Adolf, honey, say thanks to the nice man"
The point is, what will happen will happen, and some events are just pinned in the history of human kind. Trust me, I am no fan evil, but some of the greatest changes to the world and society and mentality of men came after great evil came to pass.
So would I kill Hitler? Probably not. (My late gramps, bless her soul, would kill me if she ever read this.)
But this seems like an issue philosophers might answer better than a mere mortal commoner such as myself.