mbp had a very good question in the open Sunday thread, asking "Can nice people role play villains in a hardcore game like EVE or Darkfall where villains really do upset their victims or do you need to be a b*stard in real life to be a b*stard in game?". The important part of the question is the distinction between upsetting victims and "virtual evil". When Edward Castronova once suggested on Terra Nova that only evil people play Horde in WoW, he was greeted with a mix of outrage and ridicule. To misquote Forrest Gump, most people agree that "evil is as evil does", that is an evil person is defined by doing evil acts. In World of Warcraft a player of a Horde character is not doing inherently more evil acts than a player of an Alliance character. Not only are Horde quests more often than not exactly the same do-gooder help random strangers with their errands stuff as the Alliance quests. But also in the cases where the quest text describes evil, that evil is directed towards NPCs, who don't really mind. An orc doing a quest to kill humans is not any more or less evil than a human doing a quest to kill orcs, and in both cases the "victims" are computer-controlled characters with no feelings. Their *role* in that game is getting killed, and as that "kill" doesn't hurt anyone, most people are wise enough not to read too much moral depth into whether the skin of the NPC victim is pink or green.
Even PvP can be done without moral problems, as long as that PvP is consentual. Killing another player's character in a battleground is what both players entered the battleground for in the first place. It is an agreed upon part of the game, and while it might cause some excitement (which, again, is the purpose), it isn't really likely to hurt anyone. It is arguable in how far the same holds true for games with non-consentual PvP.
But in any social environment, even a virtual one, real evil exists as well. That might be very minor, like blocking a flight master with your mammoth on purpose to annoy other players. It might be griefing others by ganking them. Or it might be bigger, like a deliberate action to destroy everything what other players worked for to make them rage-quit the game. In the Darkfall bank heist example, or in various EVE examples, people lied their way into a social structure to destroy it from within. That is evil. And the people doing these acts *know* that their acts are hurting and upsetting real people, not just some computer-controlled NPCs. Evil is as evil does, people deliberately going the extra mile to hurt other players beyond of what is required for gameplay are evil. People that are very nice in real life don't engage in that sort of behavior even in virtual worlds, and often don't even play the sort of games in which it is possible.
Of course on a greater scale of things, the maximum evil you can cause in a virtual multiplayer environment is rather limited. You can upset people, even make them quit the game, and that is obviously not very nice, and points towards a mean streak in character. But compared to causing bodily harm or killing real people, killing or robbing their avatars is small fry. Virtual world griefers are just small, mean bastards, and the very smallness of their evil acts points towards them not being a huge threat in the real world. Griefers bully other people in virtual worlds mostly because they are too small and afraid to bully people in the real world. John Gabriel's Greater Internet Dickwad Theory gives part of the answer is stating that annonymity and an audience are important factors to make people behave badly on the internet. Griefing other players in a virtual world guarantees the griefer a captive audience, his victims, and the anonymity of not being physically present protects him from any negative consequences.
Thus while I would agree that it takes an evil person to do evil things in a multiplayer game, petty acts of griefing in a multiplayer game point more towards the person being a petty jerk than him being a future mass murderer. People who are really nice and just want to "role-play evil" do that in games like Fable or Dragon Age, where they know that their experimenting isn't really hurting anyone.