JMO:
I do think CW killed SW first. She arrived home, they did have an emotional conversation. I wonder perhaps she becomes upset over the fact he informed her that he was done in the marriage. I don't think SW was abusive if she did have a reaction, she was probably stroppy (but that didn't make her a bad mother or wife or friend or person.)
She would have been mentally and physically tired. Maybe she said some stuff like. You can't leave, how will I cope with 3 kids! they will suffer without their dad here!
Everyone seems to insist she was the strong one in the relationship, but maybe that thought just terrified her.
He could have lost it, deciding that if she couldn't cope with 3 kids on her own, then he took that choice away from her totally and then carried on to the girls. I hope that makes sense.