The killer was member of a drug cartel so, while he may be unskilled with firearms, it's hard to believe he was unfamilar with guns.
His lack of shooting skill may undermine the defense claim he did not intend to shoot the victim, he was just too bad.of a shot
I keep wondering why he was not charged with stealing the gun, instead of merely possessing it. And, it seems the gun was government property -- wouldn't that also bring a federal charge of stealing government property?