That's a great question, of course, but I also always wonder about the other side of the question. Why were the Germans so into following him? Why were the Germans so disturbed? (By Germans, I mean the cultural group, which includes Austrians.)
Hitler would have gotten nowhere fast with his ideas and obsessions if a nation or two of people hadn't been there, ready to embrace and enforce some of his ideas. To be honest, I worry a little about how we (myself included) tend to want to put the blame of atrocities on individual leaders and their small groups rather than spread it evenly and include the much larger groups of people who explicitly or implicitly support the atrocities.
Leaders, at best, can tap the forces that are already in play in a society. Hitler didn't invent hatred toward the Jews, for example. While some people try to understand Nazi philosophy by looking into Hitler's early childhood, that method strikes me as short-sighted and limited.
The link below leads to a discussion of an important book about how everyday Germans, not just a handful of Nazi officers, made the Holocaust happen.
There is no way to know why a person acts as he or she does.
Clearly, many people who are not evil like to have people follow them. American politicians do this just as much as anyone else.
As far as why Hitler was disturbed, I have a couple problems with that question. First of all, what does "disturbed" mean? Are you saying he was insane? I don't know why you would say that he was insane rather than just "evil." He seems to have been able to do a lot of things (take power, make war plans, etc) that a truly insane person wouldn't be able to do.
So I think you need to rethink your question a bit. Maybe change it to "why was Hitler so brutal?" Or "what did Hitler have against the Jews?" Then the question might be more answerable, although it's still hard, if not impossible, to say why people are the way they are.
The problem is not the evil personality of an individual. The source is found in the need of german society and mentality of the German people that expected a leader to resolve a historical and economical crisis. The industrial and militaristic German civilization, which was defeated in the First World War demanded a rematch and wanted a new empire. This policy, which considered that a territorial empire is natural for a big nation, was a tradition among the leaders of Germany, although there was no moral justification to occupy nations that have nothing in common with the German people. This mentality characteristic to the big german nation, in terms of population, but paradoxically devoided of historical vision, was forced to be changed by the smaller nations for that the loss of freedom is inconceivable.