All of the factors you cited, Ace. Plus, Klydon is right on in that flashback scene--the way young Michael says to Tom, " You...talked to my father...about my future" is as chilling as the scene where he says, "You're nothing to me now, Fredo."
However, in an (alas) deleted scene just before Vito gets shot, Michael and Kay are cavorting in a hotel room, and we see--for a brief moment--a happy-go-lucky, playful Michael. The contrast with the later Michael is amazing.