Some on these boards believe Michael is a tragic figure. They're not wrong. But, nearly all the tragedy was self-inflicted. Michael had free choice at every turn, and he freely chose the Mob life, with disastrous results:

First: He was right to think Sollozzo would try to kill his father after the failed hospital attempt. He was wrong to believe that only he could save his father by killing Sollozzo and McCluskey. Ironically, an idea that Michael himself had suggested could have been modified to solve the problem bloodlessly. The Corleones could have fed the newspapermen on their payroll the story about McCluskey being a dishonest cop mixed up in drugs and murder before, not after, the trigger was pulled. McCluskey was on the take all his life, and the Corleones had all the details because they paid him. The newspapers would have given that story such headlines that the Police Commissioner would have been shamed into providing Vito with an army to protect him, to save further embarrassment. McCluskey and Sollozzo would have been neutralized without any bloodshed. At minimum, McCluskey would have been transferred or suspended pending investigation. With pressure from the Corleone judges, he’d have been indicted for taking bribes. Police would have hunted down Sollozzo and killed him "while trying to escape," to prevent him from testifying against Mac. With Mac alive, the cops would have had no reason to crack down on all Mob activities. There would have been no Five Families War of 1946, leaving the Corleones free to exact vengeance on Tattaglia. Michael could have married Kay and gone back to college.

Instead, Michael chose to kill Sol and Mac, setting in motion his abandonment of Kay, his Sicilian exile, the Five Families War, Carlo’s betrayal, Sonny’s murder, Apollonia’s murder.

Second: He could have resumed the legitimate life after returning from Sicily. He could have said to Vito: “Pop, I was wrong to distance myself from you. But I atoned: I saved your life. And I paid a heavy price: two murders, abandoned my beloved fiancée, lost months out of my life in Sicily, lost my beloved bride to a bomb intended for me. We’re quits. Oh, not feeling well enough to take the reins? Fredo not equal to the task? Sorry, Pop, that’s not my problem. Besides, you always said you didn’t want this for me—you wanted me to be a pezzanovante. Well, I can’t be Senator Corleone or Governor Corleone if I’m Don Corleone. Why don't you retire and give the family over to Tessio and Clemenza."

Instead, Michael chose to be the new Don, setting in motion Tessio’s betrayal, the Great Massacre of 1955, Connie’s widowhood and breakdown, and the beginning of Kay’s disillusionment with him.

Third: After moving to Tahoe, Michael could have retired behind the walls of his compound and invested his wealth legitimately—even putting money up-front in the legal casinos of Nevada.

Instead, he chose to hide his ownership or controlling interest in three hotels; muscle Klingman out of his interest in a fourth hotel; dominate the New York mob scene through Frankie Pentangeli; undercut Pentangeli through his support of the Rosato Brothers and their drug-dealing; and plan for a huge international expansion of his gambling empire through his deal with Roth. Results: Fredo’s betrayal; the machinegun attack that nearly killed Kay in her bed; Kay’s estrangement, abortion and divorce; Anthony’s estrangement; Fredo’s murder (and a host of other killings).

Fourth: he was “legitimate” in GFIII—but was he? He whines, “Just when I thought I was out, they pull me back in.” But he was never out. He was still a member of the Commission and influential enough to keep Zasa from rising (so Vincent tells us). He laundered his Mob cronies’ money through his “legitimate” businesses (maybe through his foundations) and cut Zasa out of his share.

Result: the machinegun attack in Atlantic City that killed all his pals and precipitated his diabetic stroke. It wasn’t enough that he became a Papal Knight: he had to dominate International Immobiliare by bribing crooked-as-a-corkscrew Archbishop Gilday, setting in motion Altobello’s betrayal and putting him against Don Lucchese, who was far more powerful in Europe. And, in an act of supreme irresponsibility and egotism: told that Sicily’s top assassin—“a man who never fails”—has targeted him, Michael gathers his entire family around him in Sicily, making them all sitting ducks. Surprise, surprise: his beloved Mary gets killed and his budding reconciliation with Kay is nipped in the bud, leading to his own, lonely death, attended by a little dog.

Michael succeeded--in turning everything he touched into death, including his own.