My two cents worth!

Michael and Kay, sentiments aside....

I reckon Michael's choices were thrust upon him

She was exposed to the gun to the band leader's head, his brains or his signature

I reckon she had an idea what she was getting into, might not have been the full extent, she knew they were crime family, about the killings and he has become like his father when she married him but deluded herself or was a turn-on for her to be connected to that different and dangerous and sort of exciting life.

He deluded himself believing he can make the family legitimate in five years

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

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

They became each other's horror but I don't think he lied to her because I reckon he, himself believed in the bill of goods he was selling Both were unrealistic in their expectations