The word "great" has a range of meanings, sometimes implying bigness (as in the Great Lakes), sometimes implying goodness and/or approval (as in everyday speech "great movie" or "great party"). Usually "great man" is used in the latter way, to indicate approval. Of course, this doesn't mean that all great men are saints. For example, though Winston Churchill is often described as a great man, he performed some actions, and has some characteristics, which lots of people would not approve; but on balance most people would agree he merits the adjective "great". On those terms, I don't see how Hitler can be described as a "great man", though of course he was a great rabble rouser and great warmonger. Your term "key figure" seems very apt.