In college, we used to play an internet version of Risk. There were six of us that lived in a four room suite sharing a common hallway. The plots, betrayals, and backstabbing were epic. You could guess at which alliances existed by watching who was sneaking into whose room, although since IM was available as a stealthier method of communication sneaking around was often done for misdirection. There was always a risk, though, that if you tried to plot over IM that the other person would betray your plans to his roommate to gain leverage or trust.
I enjoy Monopoly for much the same reasons. I love to make various financial derivatives using various characteristics of my properties. If it could be imagined, I sold or bartered it, including X-turn immunities, partial property cashflows, and dozens of other goods.
The worst was that one of them even had the ability to explain to you that it was all a strategic move to make the guy he had sided with to trust him. Evil guy would then backstab erstwhile ally, using YOU as the hammer. And of course you'd happily go along with it.
But that's not what's really evil about Diplomacy. What's really evil is that you NEED alliances just to survive, so you can't just blow these guys off if they are on your borders.
Evil.
See also, frustration.