In reading about leaders who have seized power and ruled as autocrats, particularly Lenin and Hitler, I get the distinct impression that they were more motivated by the belief that they were doing what was best for "the people" than a personal desire for power. This has led me to two questions:
1.) How do historians go about deciphering the motivations of historical actors, particularly in states where accounts are heavily clouded by propaganda and cults of personality?
2.) Are despots more often guided by a sense of ideology and "the greater good" or personal ambition, and is the former concept tied to specific societal factors? The only other "greater good" dictators that come immediately to mind are Sulla and Julius Caesar, both of whom operated in a political context much closer to that of modern dictators than other rulers that come to mind, and I can see how a world in which public opinion matters could be a major factor in how leaders se themselves.
I can tackle part of your question. In terms of Hitler, there's essentially two schools of thought that have developed since the end of WWII. It's generally referred to as intentionalism vs. functionalism (or structuralism). I'll sum them up in very general terms.
Intentionalism focuses on Hitler as the essential component of the national socialist movement in Germany, and without him there would have been nothing, from the inception of their propaganda to the final solution. Intentionalism almost turns national socialism and fascism into Hitlerism, which can disregard societal and political aspects and places too much emphasis on the leader.
This was where the functionalist critique came in, positing that it was a variety of societal factors and events that triggered certain events within Germany that national socialism could take advantage of as well as taking a closer look at the organisation of the NSDAP and the interagency rivalries and clash of personalities within the regime after the merger of the party structure with the institutions of state. They would counter the focus on the leader with the fact that he was a 'weak dictator' who disliked dealing with matters of state.
A good source on this is Ian Kershaw's 'The Nazi Dictatorship', he does a good job at summarising the plethora of perspectives on Nazi Germany. I for one favour the later school of thought, which doesn't totally disregard Hitler as a factor but attempts to get away from an almost fanatical focus on him (like did it really matter if he had one testical?).
Dont think it answers either of your questions but I hope I helped.