Caligula is not a good comparison to Hitler at all. Caligula was an emperor by right of birth, and everyone knew he was batshit crazy. It's not like he ever had supporters in the right sense of the word, or that he represented any kind of ideology, or was adored by the multitude.So much straw going around.
Hitler was abborent even in his time. Cortez was not. Simple as that, really. "Product of his time" means that the moral deficiencies of a person were common to people of that time. As the risk of continuing that Godwinning going on, is it really THAT hard to see the difference? Hitler and his actions wouldn't have been welcome in humanity in most periods of history. Cortez was a conquerer, yes, but also liberated what is now Latin American from the evil of the Aztecs.
To deNazify this a little, we could see a USS Julius Ceasar, but a USS Caligula would be obviously out of the question. We all know that Julius was far from a perfect role-model, particularly when applied to the moral standards of 2009. Caligula was never accepted as a hero, for obvious reasons.
Along that vein, we likely won't see a USS Pol Pot, or USS Stalin, or even a USS Lenin. But a USS Goberchev? Even a USS Kruschev could be possible. Of course, we DID get the USS Soyuz and USS Gagarin, so it's not like the Soviet Union is being ignored anyway.
Hitler became the chancellor by legal means, he had a lot of supporters, and when you say "he was abhorrent even in his time", the next question is obviously: to whom? He was to some... not to some others. If he hadn't been popular in Germany, he wouldn't have gotten to be the leader in the first place. And not just among ordinary or uneducated people - just as there were many German intellectuals who were anti-Nazis, there were also quite a few who embraced Nazi ideology. (German medical community, for instance - and I'm not talking just about Mengele here - contributed a lot to the genocide, developing the theories about Jews as an ill, degenerate, inherently criminal and sexually deviant race, comparable to germs and parasites, who need to be "eradicated" for the health of the nation.) I am sure that many Germans would have told you back then that he was a great man, if you had asked them when he was alive - before the end of the war, Nuremberg trials, the truth about the Holocaust became known. He was also most probably not abhorrent to his allies, or supporters of fascists/Nazism in any other countries - at the time when fascism was certainly not a universally condemned ideology as it was after WW2... And before the war, you would have found quite a few Hitler sympathizers in the countries that later became his enemies in WW2. In UK, King Edward VIII was a known Nazi sympathizer, and after his abdication, he and his wife visited Germany as honored guests during WW2 and he even met personally with Hitler. US Senator Joseph Kennedy and aviator hero Charles Lindbergh were the more famous among those Americans who were strongly against USA going to war with Hitler's Germany and who were quite sympathetic to some aspects of Nazi Germany, especially since they were anti-semitic themselves (Lindbergh changed his mind years later, after a tour of concentration camps). Heck, Hitler didn't even found the National-Socialist party (German Workers' Party, as it was known) - he was its 55th member, who was impressed with party founder's Anton Drexler's ideas, and quickly became a leader because of his oratory skills. Hitler didn't invent nazism on his own (apart from changing his party's name and therefore being responsibl for the word itself), most certainly didn't invent fascism which had developed in Italiy; and as for rampant anti-semitism, that one was centuries older and much more widely spread. "The Protocols of the Elders of Zion"? Created and published in Russia in 1903.
Hitler was no fluke or anomaly, he was very much a product of his time that gave rise to fascism and nazism - and to claim that he was just a madman hated and abhorred by everyone would be to falsify history.