Napoleon, Alexander the Great, and Hitler - they all tried to conquer the world, and failed!

• Doctor Hell, MazinKaiser

The above quote, from a 2003 OAV (non-syndicated Japanese anime), always comes to mind when I read questions with these comparisons. The mangaka who created the characters the show is based on, Go Nagai, grew up in the wake of World War II, something that he admitted influenced his notoriously violent, sexualized and generally crazy - but highly influential - manga in the 1960's and 70's - I recall one interview where he explained that he did not think that children and youth should be too insulated from knowing about the horrors of war. A related fascination with the notion of an "ultimate weapon" granting the user the power of a god (or devil), and the mechanization of war, led to him being one of the great pioneers of the mecha genre with the creation of the 1972 Mazinger Z, which the aforementioned OAV is ultimately derivative of.

That's a bit of a digression, but it should hint at the fact that the view of Hitler and World War II as a special, unique kind of evil, detached from the usual horrors of warfare, is not ubiquitous everywhere. Nor is this view, where it is prevalent, always straightforwardly logical - probably more often than the sheer death toll of World War II, Hitler's and the 3rd Reich's evil are emphasized by pointing to eugenics (widely popular outside Nazi Germany prior to World War II), antisemitism (widespread in the entire Christian world before and after World War II and the Holocaust), beliefs in racial superiority (also widespread among European populations), and cruelty partly motivated by anti-Communist sentiments also widely held outside Germany.

There are many on this subreddit more able than me to elucidate on these matters, but it suffices to say that the stigma associated with Hitler and Nazi Germany is to an extent tied to a distinct ideological need to differentiate the rest of Western society from them. This goes all the way back to Churchill's characterization of Hitler's methods as "medieval barbarism" - as opposed to the type of modern, imperial warfare it was, and which the British were so well-versed in.

I'm not going to dwell too much on these (very complicated) questions, because again, many others here are better equipped to do so, but I will link you to this earlier answer of mine where I look at some alternative characterizations of Alexander.