Hitler, just like anybody else, sinned the sins common to man. I dispute the 6,000,000 number, but let's not dwell on the numbers just now. Jesus told us that if we hate our brother, we are no better than murderers, yes? There is a continuum between the best of us, say, John the Baptist, and the worst of us, say Hitler, Stalin (killed far more), Mao (killed far, far more), and Pol Pot. We UR's are saying that all will be saved IN TIME (that is to say, ultimately). We are not saying that the monsters among us will waltz into Heaven Scot-free - they will face judgment just like the rest of us.
Otherwise, how will God ever be all in all? His view of justice and mercy is far more enlightened than ours...