Lets spare a thought for the boy to, he is going to grow up carrying this event with him for the rest of his life.
An 11 year old has sufficient cognitive powers to understand life and death. And a decent understanding of right and wrong.
An 11 year old is not a victim when he deliberately shoots and kills someone. That 11 year old is damaged /before/ the shooting. Most (western) children of that age would not be able to bring themselves to commit murder.