There are no moral codes within video games because video games are virtual, not real. If moral codes existed in video games we'd all be sent to the gas chamber, as even a single experienced gamer has killed millions of living creatures, as well as thousands of his fellow man (PvP), only rarely with complete justification.
If we follow this logic it might be considered a crime to delete code - the only way to kill Joel from The Last of Us, to well and truly annihilate him, is to delete his code along with all backups so that he no longer exists, as well as erase the memories and creative foundation of Joel within the Naughty Dog developers so that Joel can not merely be re-created. What PLAYERS are doing when they "kill" a creature within a video game is to remove a creature from that particular game phase and except for respawns the entire future of that specific game phase - that creature continues to exist in all other game phases, including prior save slots of the very player who killed the creature.
It's far more murderous to delete a game from one's hard drive, which "kills" the entire game code from the system, than to shoot and "kill" a monster within the game. The only way to truly kill is not to play.
Players aren't so much killing creatures in games as bringing life to them, as otherwise they would be idle and only potentially existent, as raw unexperienced code not even processed by the CPU, video card, and sound card, and loaded into memory, merely taking up hard drive space.
Video games are a lot like gods in this respect. Gods only exist if they are believed in (played), and when they are forgotten (deleted), they disappear. A god would much rather be part of the pantheon and murdered by Zeus than not exist at all, and if we could ask Joel if he would rather be played by a bad player who gets him killed several times prior to deleting the game one quarter of the way through or not be played at all he would take the former.
If this study is being honest and players actually feel guilt for removing a creature from a game phase then a follow up study could be conducted to determine whether gamers are insane.
In contrast, it makes some sense to feel guilt for removing fellow human players from a game phase, especially friendly-fire situations during co-op. Since people playing a game want to play the game and not wait for the next round or wait for a respawn and then undergo travel time to return to their desired location/context within the game, it actually injures the well-being of a person to kill his avatar in a game phase, therefore guilt is a natural reaction. A small amount of guilt only of course. After all, it's just a game.