Very compelling viewpoints from both TB & LP...

I do agree that Michael was seduced by the life - although it was a slippery slope, one that quickly became ever steeper & slipperier with each new step. For that reason I feel that, although Michael in theory could have walked away at almost any time, in practical application the real costs in doing so became greater, at nearly an exponential rate, with each new event that transpired. After Vito's passing, that potential cost included the complete annihilation of the entire Corleone family - a fate that, of course, would have been utterly unthinkable to Michael.

Looking back on it now, I gather that it was never completely realistic for Michael to live at the Mall as a civilian, knowing what his father & his brothers were up to on a regular basis, and yet expect to never get wrapped up in it, regardless of events. Sooner or later, something was bound to happen that would blur the lines between the business & the personal in a way that would likely pull Michael over the line - unless he had wisely anticipated it, and was very well prepared to take effective counter-measures against it when it finally did occur. And yet there is little in the GF I movie, at least, to suggest that such preparation ever seriously took place.

For Michael, the seeds of transition from civilian to Mafia don began on the night he learned of the attempt on Vito's life. His anguish was palpable - so much that it seems to have already begun to drive a wedge between Michael & Kay, as evidenced by Michael's aloofness toward Kay during dinner at her hotel room soon afterward.

But the real turning point for Michael was the second attempted hit on Vito at the hospital later that night. Something about that experience changed him. By the time of the subsequent meeting with Sonny & Tom about "the Sollozzo question", Michael volunteered to hit both Sollozzo & McCluskey - as Turnbull has rightly pointed out - not because it was the only possible option to ensure Vito's security, but because deep down, Michael wanted to do it - thereby turning one of the most famous lines in the entire film: "It's not personal, Sonny. It's strictly business." into one of its most poignant & powerful ironies.