When I read stories like these, I think I try to convince myself the parents must have been mentally ill. I can't wrap my head around any mother or father doing something like this as an act of revenge. Maybe it's weird, but I find a small comfort believing the father was sick and maybe thought he was somehow "saving" his son. The alternative, that his need to punish his ex outweighed his love for his son, is too heartbreaking to accept.
I also find it more upsetting when the child is old enough to understand what is happening. I can't imagine the fear this little boy felt.