Stupid, stupid, stupid adults!
"Sounds like he had some real anger issues relating to his parents' divorce that were never addressed. Folks think that kids are so resilient, that they can 'get over' a divorce easily, then accept it when a new baby comes along. The kid suspects he's lost his father's love, and now that's reinforced by the presence of the new baby and the father is so happy about the baby that the boy thinks his father will never love him again. So he does the only thing he knows to do, remove the rival for his father's affections. "
I think you slammed that nail right on the head and to the hilt. I don't think there's a need to add anymore than precisely what you've said.
You have to admit to a difference between a child who is having problems dealing with the breakup of its parents and a child who would murder a 7 month old infant.
While the parents are to blame, I'm of the opinion it has more to do with their lack of discipline of the child, not teaching the child moral values, right and wrong and the difference between reality and fantasy. In other words, failure to parent.