There will be many answers to this. The massive consolidation of power that enabled Hitler to pursue ends that would defy the limits of how terribly human beings can be to one another is one reason why he embodies the idea of an evil political leader. Hitler is one of the few historical figures who is able to reconfigure our understanding of both political and personal cruelty. Few others can accomplish such a dubious distinction. His systematic campaign to literally silence his perceived "enemies" and dissolve the institutional checks to centralized power represents the accomplishment of both levels of cruelty. In the end, Hitler becomes the symbol for which all have to be vigilant in guarding against and, in this light, his presence has to be considered evil for individuals are more attune to the mistakes in conferring power to leaders.
This is an extremely difficult question, but one of the reasons was obviously the Holocaust and the systematic attempt to exterminate the Jewish people and many other "undesireables." But the difficult part of the question is why this was emphasized over (and continued to be) other despots who've killed as many or more people than Hitler did, Stalin for one.
Some would argue that Hitler was portrayed as so evil because his country lost the war whereas Stalin was helping us win so we were not as quick to villify him. He also was not as public nor in some ways systematic about his mass murders, though it is likely that he killed a far larger number of people within the Soviet Union.
I tend to think that it is partly the incredibly systematic nature of the Holocaust and the incredibly organized way it was carried out that makes it so uniquely terrifying.
All you need is to know two things: the Holocaust happened and World War II happened. That is why people think that Hitler was one of the most evil political leaders ever.
You can clearly blame Hitler for World War II (although the Treaty of Versailles is also to blame). It is his efforts to go back on the Treaty of Versailles and to increase German power that led to the war.
The Holocaust is clearly the greater of these two evils. Here, Hitler caused something like 7 million people to be murdered. Most of them were murdered because they were Jews.
So Hitler essentially caused a bunch of people to be murdered and also caused one of the bloodiest and most destructive wars in history.