Alex and I were discussing recently, and it came up that I'm remarkably OK with PCs working counter to each other (I think we may have been discussing Tim's traitor behavior in MnM). Alex posited that this was a result of my wargaming experiences, where these same players were enemies to be crushed beneath the volume of fire from my laser cannons, and the shattering of their hulls was cause for rejoicing. That may be part of it, but I think that more important is the fact that I have generally had positive experiences with such play; Jared kept important secrets from the party in my first Traveller game, Asmir plotted against Jared's wizard in the second season Tim's game, Oknir played with and against us in the third iteration of that game, Fjolkir/Mavrilith killed a fellow PC and got away with it, and Mordecai worked at cross-purposes to the rest of the party in Alex's Trav game. None of these incidences of 'playing against' resulted in much wailing and gnashing of teeth, nor the ending of games, and in fact I would argue that definitely Jared's playing against the party made the game more fun. The others were a bit ambiguous; I think in these cases, the more that the treachery is kept secret, the less fun it is for everyone involved.
However, more generally, I realized that while playing MnM, I kept kind of urging my fellow players to pursue courses of action (riding disintegrator dragons and using the incorporeal telekine as a scout through hazard-sealed doors) which might be, ah, hazardous to their health. I guess part of it reminded me of Drew's trickery towards Sheng and Jason during their first session, where he convinced them to poke the skeletons and open the cursed chest. I started to realize that at some point along the line, it went from "treachery is OK" to "Even when I'm playing, I'm kind of gunning for the other PCs a little." And I'm not entirely sure how I got here, or how I feel about it.
I think playing Asmir planted the seeds; I had never plotted to kill a PC before, but Asmir put me in a position where I had a couple layers of contingency plans to eliminate the entire party (I even appointed myself Party Cook in order to better poison the stew). Before that I was in a very tentative "sometimes, if you do dumb things and get unlucky, you may end up dead" position as a DM. Mordecai in Alex's Traveller game contented himself with smuggling dangerous goods without the rest of the party's knowledge, and plotting the downfall of interstellar megacorporations with the assistance of a rogue AI in his free time, but never went against the party's interests (and in fact was a primary provider of income). Fjolkir was out to kill the dragon at any price, and ended up burning down a city and killing a fellow party member to do it. That was my first (and hopefully only for the foreseeable future) PC-on-PC kill (but not the first of that campaign, as Oknir killed the paladin off-screen), and I did try to find ways around having to do it.
And then I ran ACKS, and that just downright desensitized me to PC death. It's odd, how I never realized that 3.x and friends lacked good rules for handling death and character replacement until I ran a system that did have fairly comprehensive rules for it. But to some degree, my brain is still in ACKS-DM mode while playing MnM, and it's... probably not the best of things. Perhaps it's a sign that it's time to sharpen the ACKS again, I guess.