Prior to the assassination attempt against Don Vito, Michael’s whole trajectory had been a sort of WASP-ization, a gradual migration towards the “respectable” law-and-order Anglo-American values incarnated in Kay. He explicitly rejected - and told her about this - his family’s recourse to criminal or anti-social shortcuts. And it seems this cultural journey of his was a foundational element in their relationship.
When he meets up with her after returning home, he’s a completely different person. When we contrast the way he speaks with her in front of her school versus the way he’d spoken with her at Connie’s wedding, we almost get the sense he’s been brainwashed. The bridge from her apparent hesitation to her capitulation does not seem clear.
Why does this new Michael not scare the living daylights out of Kay? Was she actually enamored with that tiny bit of the “exotic,” Latin “other” in Michael all along and his transformation just made him all the sexier to her? I’m not sure I read any evidence of this.