I suspect you're probably right in that playing evil lets the 'dark' side of our persona free to do things that would be frowned upon in real life. I must be suffering from perpetual 'nice guy' syndrome in that I can't even be really mean to someone in the game unless they really, really deserve it.

But I also wonder if the game is not engineered to give that dark side a little more free reign to be wicked and to frankly be more powerful on an individual basis than the good-aligned.

The 2 PC builds with the greatest potential to PK on an individual basis are probably anti-paladins (i.e. Cabdru) and necromancers (Entropy lich). Even the rockin'est paladin couldn't stand toe to toe with characters of that ilk. They might be able to escape alive, but even that would be a challenge. I realize these are exceptional examples, but you get the gist. I think it is that potential, whether realized or not that attracts the power-pk'ers to the dark side. (For some reason I want to say 'Use the force, Luke').

Anyway, just some idle musings.