Never said any such thing.
First, child onset mental illness with auditory hallucinations ie "voices" is actually really rare.
Second, I read several stories on this. As soon as granny got home he knew he was gonna be in trouble because the school had already called. The boy calmly went in the other room, got the gun, shot granny, and then when the police came lied and said he had nothing to do with it.
Using my common sense, I don't think he heard voices instructing him to do anything aside from his own fear of getting that good a$$ whoopin' granny was probably going to give him.
This child, like many other black children, unfortunately lacks a stable family structure, good parenting, and hasn't been taught to reason or control his impulses. He needs help surely, but mental illness isn't the issue.