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.