Games have rules. But besides those rules there are often conventions and social norms, which are not governed by the game rules itself. The behavior of players and supporters of a game is often influenced by factors outside the game rules, by how clubs are organized, how tournaments are organized, and so on. But that separation is less obvious for online games, as the game not only sets the rules for the game itself, but also organizes how the players come together. Social issues like cheating in online games can have technical solutions, like Diablo 3 is attempting to make their game cheat-free by keeping all information server-side. And changes in the game engine on how players meet, for example the introduction of the Dungeon Finder in World of Warcraft, have profound social consequences on how players interact with each other.
Yesterday Helistar remarked in my post about Rohan's experiences with other players in raids that these experiences "had absolutely nothing to do with the game". And for the reasons listed above, I beg to differ. I think that how nicely or horribly players treat each other in a MMORPG is very much influenced by game design, because the game design not only covers the rules of the game itself, but also the effects of how players interact with each other. If you took two otherwise identical MMORPGs, but made grouping give much more bonus xp in one of them than in the other, you would over time observe very different player behaviors and attitudes towards grouping. Blizzard is pretty obviously constantly social engineering player behavior in their endgame by fiddling with nerfs and various systems to distribute epic gear, with guild perks, and RealID tags.
Thus the developers cannot be completely absolved from any responsibility for players behaving badly. One guy behaving badly is probably a problem of that one guy. But if similar modes of bad behavior are widely reported in many groups or guilds, we need to look at what parts of the game design favor that sort of behavior. I am sure that Blizzard thought very carefully about how to distribute loot among group-members in Diablo 3, because by having a game design which attaches real money value to every piece of loot, there is an obvious risk of fostering bad behavior among players. I've seen the negative effect of a real-money player economy very clearly back in the days where I played Magic the Gathering Online. It turned what was a fun game into a cesspool full of card sharks.
But besides the dangers of game design fostering bad behavior, there is also an opportunity of better game design fostering better behavior. I once was positively surprised of receiving a very nice welcome from more experienced players in some Asian Free2Play MMORPG; it turned out that this game had a system where experienced players would mentor new players, and in return receive rewards based on how well the new player was doing. The first Asheron's Call had a similar system of lieges and vassals, which gave the game a completely different social dynamic than other MMORPGs. Players of MMORPGs react strongly to incentives and rewards, and a game in which you are rewarded for helping new players ends up with a very different community than a game where it would be advantageous to gank them.
Of course game developers already have realized this. There has been a constant development over the last decade to create game rules which prevent bad behavior. We went from simply "play nice" policies to games in which it became increasingly impossible to hurt other players. Today it is hard to imagine that there were games where you could actually "kill steal" or "ninja loot", getting the xp and loot rewards from a mob somebody else pulled. The terms are still used, but now describe much less critical modes of behavior. I can only hope that in the future this trend goes from removing possible interactions between players towards enabling positive interactions between players, and rewarding them.