This has nothing to do with "impluse", this was a premeditated crime. He waited until his father was at work and his sisters were out of the house. He covered the gun with a blanket so it wouldn't be seen and went up to the sleeping woman and shot her in the head. He then went outside to get on the bus, but first disposed of the weapon by throwing it in the bushes. Not to mention he'd been threatening the family in the weeks preceeding the murder.
Yeah, not a mind that's capable of knowing the implications or severity or atrocity of his actions at all.