I wonder if the child's illness played any role in the SW's decision, as it's a hereditary disease, and to be affected both parents have to be carriers of the gene causing it. Were there any young relatives of SW having died of the disease recently, maybe him having seen the effects of the disease? If so, he might have felt guilty of having passed on the disease to his son, and knowing how it was going to affect the child. Add to that a divorce, and who knows what kind of thoughts SW had. It's not an excuse for what he did, but it can be an explanation to why, was it to hurt his ex-wife or did he see it as a twisted way to "save" his son from the effects of his disease.