While I defer to your expertise, I have to say I find it a bit difficult to believe. Being in an online game just changes the form of interaction between people, not the fact that they're interacting. (We couldn't, for example, say "Oh, X is a jerk over the telephone but s/he's not like that in person at all".) These players know full well that there was another human being at the other end of their misbehavior.
Whatever the online analogue of "in vino, veritas" might be, I would be willing to bet that it's true and that these players are actually revealing a significant aspect of their true nature.
They just saw UO as a different kind of game as other folks did, something more akin to Quake where it was no holds barred PvP. There were multiple folks I knew who couldn't understand that someone wouldn't view the game that way.
One of the games that I've played over the years that specifically embraces this is Kingdom of Loathing.