My take, for what it is worth!

Kay knew Michael had gone from “That's my family Kay It's not me” to It is me when they married and was happy to enjoy the spoils of the unholy and evil Sicilian thing for five years, the original time frame for “legitimacy” and another two years [and another....] until the Tahoe shooting, then learning at the Senate hearing that Michael had personally murdered in addition to ordering murders

Originally Posted by mustachepete
Originally Posted by Turnbull
I've always contented that Michael had choices at every stage of his life, and always chose the gangster way.

Yes. Moreover, I don't think that there's anything, in either the book or movie, that hints that Michael thought that there was anything wrong about gangsterism. It was old-fashioned or inefficient, but not wrong - or at least not any more wrong than politics or business.
Michael's “choices at every stage of his life” was catch-22 and in a way “thrust upon him” and 'forced' to “choose the gangster way” under the circumstances

among others -
  • Michael murdering Sollozzo and McCluskey to protect Vito We saw Michael's mental anguish on his return from the bathroom with the gun
  • Vito got Michael to re-establish the Corleones' glory, standing, reputation etc. to do all the dirty work, thus leaving a murderous legacy for “I never wanted this for you” son Thanks! Pop
  • Michael initially underestimating Roth until the Tahoe shooting

Michael must have thought that gangsterism was wrong Hence he wanted to have nothing to do with their family business
Though later Michael conveniently deluded and justified gangsterism was “not wrong - or at least not any more wrong than politics or business”

Once a common Mafia hood always an evil murderous monster indeed However
Originally Posted by Evita
Once Michael took over, it was not easy just to walk away He was a powerful man with responsibility for others