Not just Michael, but Vito, too.
I was only referring to Part III, in which Michael's struggle reached a culmination. Vito did not figure into the plot of III in any direct way. I never got the impression that Vito searched for redemption with God, as Michael did in confessing to the future pope in the garden of the Vatican. We are shown nothing about any of Vito's fear of God. He remained a localized strongman whose sense of self-justification was as a family and community boss and protector.
Michael, however, took on building up a national entertainment mecca, assumed national coordination with the east and west coasts and Cuban outposts of the mafia, and manipulating the global finances of the Vatican. Vito was never shown killing his own blood nor having searing regret over what he regarded as necessary evils; even when Sonny was killed, he regarded him as a reckless hothead (to Tom Hagen: “I never thought you were a bad consiglieri—I thought Santino was a bad don”), and his concern was mainly to shield his wife from excessive grief (“I want you to use all your powers and all your skills. I don't want his mother to see him this way...”).
Michael, on the other hand, suffered enormously from feeling trapped in the life he originally was not intended for, and the death of his daughter and resulting permanent estrangement from Kay because of his rage- and ego-driven life choices. He lied to his blood constantly, repudiated his ultra-loyal consigliere Tom Hagen, and killed his own brother. His moral conflict and guilt were profound. He could not be what his father had been because of his overblown ambitions and the mafia's unexpectedly massive expansion, which pushed him in over his head and his tortured heart. As he had said to his mother, “Tempi cambi’.” Times change.
You are obviously a student of the series. I would add that Michael didn’t remember his own instruction to Kay . . . that Senators have people killed too. The idea of going to Vegas was to launded the entire Corleone operation and go clean. But now, instead of some other Italian families to contend with, there were U.S. politicians, and at the too level, Old World Old Money bankers who STILL didn’t give respect. You take them on because there is always one more level of power to achieve go get that elusive “security”, and you ruin yourself AND your family in the process.