this probably not all that helpful, but here's some thoughts:
it's hard to make judgments about what goes on within a game, because, well, it's a game, and players do things to each other that they wouldn't in real life, and treat each other in ways they wouldn't otherwise, because, well, respawn, and in real life, points, XP and w/e are meaningless.
but if you feel guilty about doing something, it usually means you shouldn't have done it, and/or you might ought to apologize to someone.
i play mostly single player games so i haven't put a lot of thought into ethics in multiplayer gaming =p
but i think as Christians we ought to show more respect for other people than the world does. if scamming people out of items is a regular part of gameplay, then i wouldn't worry about. i mean, i lied, cheated and stole my way all the way through Skyrim, but i wouldn't lie to or cheat other people in a MP game. but in some games looting & stealing from others is a game mechanic. if your corpse drops medpacks and i need medpacks, yeah, ima drop you.
if it bothers your conscience to do something though, it's like sin for you, so avoid it to keep a clear conscience. as a Christian even in games, we should try not to offend people, as much as is reasonable.