You have two prisoners that are each offered a chance at freedom. They are given the choice to either rat out the other or remain silent. If both rat out the other, they both get a longer sentence. If only one rats out the other, they get a much reduced sentence. If they both remain silent, then they get something in between.
Obviously a "psychopathic attitude" would be to consistently rat out the other, despite it also being the only option that has a 50% chance of an increased penalty/sentence. It's not as simple as me-first, there is also logic and planning involved in how to get the greatest gain. This is why you get some players that will help others out, become part of the group, then take the group to the cleaners (e.g. steal all the stuff in the guild bank or whatever).
What nobody seems to be bringing up here is the concept of disassociation that all people have. One reason that some people road rage and scream at other cars is that they do not see that there are other people in the cars, they only see the vehicle. Likewise with some in-game avatar, there is no personal accountability for actions, no concept of personal punishment or social rejection. People behave much differently when they aren't being judged by others in person. There may be some lack of apathy in some players, in others maybe they just genuinely enjoy being a jerk when they won't get their face punched in.I believe that this is a big reason why we get griefers and in-game chat e-peen measuring contests, it's all a sort of virual posturing because there are no repurcussions. You take the same folks and put them in a room and I can all but guarantee the 70-lb 12 year old won't be mouthing off to the 300lb biker.