In fairness, I believe it was Vito who sowed the seeds, leaving a murderous legacy and set Michael up for his own destruction

Vito could have cut their losses and walked away from Mafia but he didn't because he wanted, needed the Corleone family reputation, standing, history etc. to be restored, leaving a murderous legacy for Michael

Vito planned all the baptism murders including making his daughter a widow for Michael to carry out after Vito's death

Once Michael took over, it was not easy just to walk away He was a powerful man with responsibility for others

I agree the whole "going legitimate" plan was one that Michael and Vito cooked up as Nevada move was work in progress when Vito was still alive

Michael had the added burden of trying to make decades old Mafia business legitimate
Michael and Kay were unrealistic that such a feat, near impossibility could be accomplished let alone in five years

If Kay hadn't become Michael's horror perhaps they could have salvaged the situation as best as possible
Perhaps Anthony would have become a lawyer which means they would not have been at the Opera and Mary would not have got killed

Even if Kay had been stewing and it never ends, Tahoe shooting, senate hearing public revelations, evasive legitimacy, she never realised how difficult and unrealistic to legitimize the Corleone empire

She never understood nor supportive that he was a top Mafia boss, trying to become a "legitimate" businessman

I also believe Kay did not know that Michael had personally murdered until the senate hearing Perhaps that was the final straw
When talking about sick Vito, she said "But you're not like him" let alone personally murder