I agree with Goombah and Flora. Michael was totally ruthless, and no consideration would have stopped him from whacking someone, no matter how close, if he felt threatened or needed revenge. I bet he contemplated whacking Kay, but pulled back when he realized that his beloved Anthony and Mary would find out eventually and never forgive him. And, as we learned at the beginning of III, he entrusted their education to their mother. No doubt a practical consideration on Michael's part (left him free to do Mafia business without having to worry about shielding the kids).