<snipped>
DS, imho, had the most to lose in this scenario. She only has one child. Her child is critically injured and perhaps already brain dead at this time. The woman who she sees as responsible for the break up of her marriage (although I think it was over long before JS met RZ), is now who she sees as being the one responsible for her sons accident. (Whether she was or not actually responsible). I am sure there was also guilt involved, as she could not be found for a time and that she was probably quite content to have Rebecca in charge of watching Max at times. She also stood to lose monetary security, (as curiousjo brought up), which was probably much lower on her scale of importance than Max, but must have added to her stress.
JS, a man of power, who always, even if he had to fight for it, gets his way. This has been true in business and in his personal life. He had been married twice, who knows when he met DS or if he was still married to ex wife #1 at the time. If something or someone is in his way, eliminate the problem. If he felt that Rebecca was responsible for Max's accident, I think he would have acted, perhaps violently, before he even took time to think it out. I think that if DS was emotionally distraught, he may have gone to the house, to talk to Rebecca to get more details aboit the accident. Either that, or when he found out Max was brain dead, he may have snapped. I think JS had the opportunity and the means, being stronger than Rebecca.