I think that in spite of his words to Kay and his entry into the casino business,Michael had no intention of going legit after the killing of Sollozzo and McCluskey. At first,granted,he was a naive innocent as far as the reality of the Family business.

As things progressed,he transformed into the ruthless,lowlife Mob Boss,and I believed he enjoyed every minute of the power and fear that he generated.

After Sonny and Appolonia got killed,was the massacre of the Five Family heads really necessary? I mean,I know Barzini was going to make a move,but wouldn't it have been enough to just take out Tessio and Barz? The other Family Bosses were not a threat to Michael,except in his head.They could care less about Barzini,and just wanted to get back to business.

Whether he would have continued the Corleone involvement in narcotics or not,the other Families would have realized that a new, strong Boss was running things,and would have honored Mike's decision.

Same with Fredo and Hyman Roth. Both were in effect neutralized,but Mike had to go the extra mile and kill them out of some warped sense of honor.

I can't help thinking that if Tom Hagen was in part III,he would have wound up whacked too.

Bottom line,Mike loved being a gangster,and despite the opportunities he had to really get out and go legit,he took none.