How is it possible to be a psychopath in a game? This and other research are based on the premise that video games contain real violence. No game has ever contained true violence in this sense, which is why violent video gaming behavior doesn't lead to the harm that real psychopaths cause in society.
The only way to act psychopathic--doing actual harm to another human being with true apathy--in a video game would seem to be through communications between players inside the game, where feelings could be hurt. It would be hard of course to separate psychopathic communicative behavior from other common factors like immaturity, inebriation, gaming cultures, etc. That should probably be the real focus of these kinds of studies. Another interesting study might be to study actual psychopaths, pulled from corporate environments or the like, and seeing if/how they play games differently from non-psychos.
This is why murder in games as a measure of sociopathy is a red herring. The real crazies are the griefers, the ones who gain enjoyment, with no other tangible benefit, from knowing they are doing harm to real people in the form of wasted time or belittling. It's hardly limited to gaming. Look at Wikipedia. Sometimes people vandalize because they have a petty axe to grind, but other vandalism is just totally pointless, like replacing entire paragraphs with the word "penis". I would even consider some graffiti, like the Chinese teenager writing "Ding Jinhao was here" at the Luxor Temple, to be sociopathic.