Actually just a small note - the 'difference' between Hitler and other 'tyrants' like Genghis Khan and Alexander is that Hitler lost. That's really it. Genghis Khan? He brilliantly and ingeniously built up the largest continuous land empire in history. Alexander? He conquered and conquered and was remembered as a great legend not only in the Mediterreanean, but later in Europe, the Middle East, Central Asia, and even as far as Indonesia. Stalin? Well, he killed even more than Hitler, but he was the 'good guy' in World War II, and by his death Russia was a superpower competing with the US. Mao? Well, I really hate the guy, but unlike Hitler he did survive and he still garners some respect in China nowadays.
Point is, Hitler may or may not have been a great leader - that's not the important thing. The important thing is that he lost. Not only did he lose per se, but he also has a very, very, very negative image attached to him, one without any redeeming qualities. Genghis Khan, at least people consider him a brilliant leader even if he was ruthless. Cleopatra, yeah, she 'lost' and killed herself, but there is so much of an exotic legend surrounding her. Joan of Arc, she 'lost' and was executed, yeah, but she's a national hero of France.
Not that I think Cleopatra and Joan of Arc should be in, but these are the reasons why I see it that mass murderers like Stalin can be in while Hitler can't.