I don't see why. If he's indeed doing it over the net and not by finding out who the players are in RL and having a buddy break in to drop some cardiac arrest poison in the players mouth, then it could be either extremely difficult or not difficult at all. Obviously it's a huge weakness in the system, both the GGO game and the headgear hardware, to be exploited. Once the method is established, it not necessarily any harder than summoning a rare gun by cheating. Only nobody's looking for any injected commands that kill a player.
Also, I don't see how a player who couldn't even get a gun normally would go around killing others for thinking they are so mighty players. Death Gun would be motivated by game envy, huh? Couldn't get much more pitiful. I really hope that's not the case.