Don Vito Corleone could justify in his mind why his criminal life style was necessary. Without him finding his "destiny" he would have been powerless, "dangling on the strings" himself. But he was comfortable with himself and remembered where he came from and that is where his true strength as a Don came from.

Don Michael Corleone never knew the powerlessness. He knew life, only from a position of strength and wealth. He never knew what it was like to be under the thumb. Therefore, his blind ambition was to free himself and the family from the illegitimacy. Don Vito knew that for Michael to be a success in the legitimate world he had to be kept clean, free from the family business. But he acted to defend his family by killing Solazzo and was sucked in. Michael's mistake was he didn't realize what a tangled web is weaved once he entered the illegitimate world and how impossible it would be to extract himself from it. His execution of the heads of the "Five Families" was,to me, not revenge but an attempt to spring himself and his organization from the web; to do away with the forces that would suck him back in.

Michael probably could have been the indisputable best Don, if he had resigned himself to a life in the illegitimate world. But that wasn't his nature and therefore, he was never comfortable with himself.