As an intelligence analyst I worked with generals and admirals a lot and found them to be much nicer than their lower-ranking counterparts.
Once you’re a big bird you don’t have to work your way up anymore. Hence you can be a little less cutthroat. I do suppose though personality has a big part in it. If you’re a real hardcore asshole as a lieutenant, you could be a real huge asshole as a general, as a person. The ones who know when they need to be a hardass and when they don’t, are probably the ones they really want to get up there in command cause they can work with people the way they need to be worked. Some need a hard hand, some can get by on a looser grip and do excellent work.
Also, it depends on if you’re really a military guy, or a political guy. The dicks are the political guys.