Hitler is not evil if you consider the concept of 'supernatural evil' i.e. state that Hitler was somehow motivated by Satan to do what he did or that he set out to do evil things. I think Dawkin's point here is that he did what he did for what he considered to be perfectly rational and logical reasons to do with the survival of his people. Many world leaders have been in similar positions before, though most of them would never go so far as Hitler did.
I suppose one question you have to ask here is: If you were put into the exact same position as Hitler was, would you make the same decisions?
And what about the decision that led to the dropping of the atomic bomb on Hiroshima? Is that evil also? I think many see it as an essential step in the ending of the war in the Pacific but it was still a morally dubious act.
Note, I am not in any way justifying any act that Hitler made during the war nor providing an excuse (if anything saying that his actions were motivated by supernatural evil is the excuse). What he did was evil by a given definition of morality but it was not because he was the devil or some manisfestation of supernatural evil. He was just a man who made some incredibly bad decisions and had to live with the consequences. I think this is the point Dawkins makes - if you accept that there is a god then you have to accept that there is the devil (or some other manisfestation of supernatural evil - every religion has one...) and therefore you can attribute anything 'bad' to that rather than resting the responsibility on the heads of those who are actually responsible.
And now this debate has been well and truly Godwinned
As for smallpox and religion... I know that Dawkins is a big proponent of meme theory and does consider religions to be massive 'memeplexes' (collections of memes with a linked theme) which are passed down from generation to generation and spread like viruses.