She might have even said "I know what you are doing", or "I'll ruin your life", or something like this, if she was angry. He might have planned it, or he might have felt that he was cornered by SW, and there was no way out, and started planning. But I am sure that he did not snap, that he had entertained the thought, although how spontaneously everything happened, I am not sure of.
I am wondering if there should be an advise to all women who are suspecting infidelity but have not discussed it with their spouses, or suspecting that husband is gay, not to discuss it one-on-one, but do it in a safe place, such as family therapist's office, and confront the spouse there. Or any other third party, but not at home alone. Seriously.
I wonder if he ever dared to kill the family if he knew that the secret was known to an external source.
I wonder if SW's mistake was, she was trying so hard to portray a happy family, be it because of her sales, or some serious perfectionism, that no one knew what really was going on there, and he thought he could get away with murder.