I think some of Michael's actions, his emotional deadness, his mistrust of EVERYONE--Including Tom, who had NEVER given Michael any reason to distrust him--Shows a Don who wasn't operating with his right mind.

Consider Vito's reaction to Sonny being killed, which is much more emotionally traumatizing than what Michael went through in Part II: He doesn't become an emotional shell or lose his humanity, he reasonably arranges a peace summit, figures out just who his enemy is, and then begins quietly formulating vengeance with Michael. And he is just as warm as before, and dies happily and at peace, playing with his grandson--Perhaps happier and freer than he ever was.

Michael on the other hand treats Kay like a non-person, a persona non grata, when she reveals she had the abortion. Even before this, he's become so detached from his (blood) family that he has Tom buy Anthony Christmas presents--Like any cold corporate executive, he isn't even buying his children their Christmas presents but is pretending to; He plans to have his own brother killed; He humiliates Tom publicly and distrusts him without reason, even though Tom did his job and kept everyone safe while Michael was in Cuba; He has Pentangelli killed despite Pentangelli being just as manipulated as he was by Roth's machinations; He basically takes Kay's children away from her and has her sent away like an old shoe; He has Rocco, his top Capo, sent on a mission which both know is really a suicide mission simply for revenge, which isn't even worth it--Roth has lost and is sickly and will probably die soon anyway.

He just seems too overruled by his emotions, by the end making him a worse Don than Sonny I'd argue. Too ruled by his emotions to make any rational judgments. Even Tom notes this. Sonny was set up and his temper was used against him. He was unlucky where Michael was in surviving an attempted hit. But neither Vito nor Sonny lost their souls. Both ordered murder and committed it, but they never became monsters, feared and loathed even by their blood.

I truly believe that after Fredo's treachery combined with Kay's abortion, Michael had a nervous breakdown. Or some sort of major psychological collapse, which turns him into an emotionally numb, paranoid, merciless, uncaring machine of a man. He has become "comfortably numb."

And I think the last scene, where his mind goes back to that fateful day in December 1941 when he joined the Marines, while both the blood and crime family were whole and at peace, reflects that even he knows this: He's lost himself. He's lost his family. He's lost his soul. He won the battle against Roth, but lost the war for his own goodness. And with the publicity of the Senate hearings, he's lost any hope of ever being a legitimate man.

That flashback has a certain poignant eeriness to it. Four of the people at that dinner table are dead: Sonny, Tessio, Fredo, and Carlo. Three of them are dead by Michael's orders. And Vito, just off screen, is dead--His life and legacy hanging like a heavy shadow behind Michael throughout the whole film, contrasting the beloved, kindly and respected Vito with the empty, bitter, feared Michael.

And the saddest moment of all: Fredo is the only one to encourage Michael to go his own way, and in the end, Michael has Fredo murdered with a bullet in the back of his head and his body thrown into a lake.

In Michael's obsessive quest for legitimacy, he lost everything.