There's a comment in the article about how the game brings out people's "true" personality. I disagree with that, as I think that optimizing your score in a game needn't/shouldn't have anything to do with personality, but for whatever reasons it's a common reaction people have when playing the game. "Oh, this person is truly truly horrible and I never realized until now." That kind of thinking generally leads to unrecoverable breakdowns in real world relations.
I think that's more or less it. I was just playing to win. Although the manner of my betrayal may have been a factor. While we were allied, my friend insisted on being the one to deliver both of our orders himself. When I broke the alliance, I allowed him to deliver the orders that we worked out together, but then I secretly delivered a separate set of orders for my own forces with a note saying that they were to countermand any other orders that might be received. It was kind of underhanded, but so was his insistence on delivering my orders (since doing so would allow him to unilaterally betray me), so I figured he had it coming.
I love the game in theory, but I felt too bad after backstabbing people. (So if we ever play, trust me! :-) )
It was shocking to realize how wimpy I really am.