Hell, During the one MP game I took part in, I was practically backstabbed (indirectly, as there was some confusion) in a manner that ultimately led to me being knocked out of the game (I'd been reduced to my capital, a few mages, pretender and under siege). At that point I went AI, but at least I had managed to weaken my opponent enough that ultimately the person who had been the most helpful to me won the game. And he wasn't even my official ally.
There was some pretty intense plotting going on at times even while there were all kinds of supposed alliances going on. That part of the game was actually great fun.
I don't hold any grudges from that game and I'd play with them again any day, aside from the one person who went AI at the first setback when it was nowhere near crippling.