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.
I think the kid's been twisted by the selfish actions of his parents. There's no telling what another kid would do in the same situation, but it's obvious there was a failure to parent, to show love, and loyalty.