Technik4 said:
If I wanted to, I could've written a biased little scenario like yours to get people to agree that I was wronged.
That is NOT the point of this thread.
Your original post was nice and unbiased. But his point should be well taken: you were acting against the stated intentions of the party to the point of killing a helpless character, much like the hostage-killer in Cadfan's example. He tries to remind you that Mossimo's character is reacting to a situation that is getting out of hand, where he sees someone clearly betray the goal of the party, and he doesn't know what that person will do next.
Was the CdG necessary? Absolutely not. A little Use Rope, a little Diplomacy: the party would have gotten things sorted out with no death on anyone's conscience. Was the CdG expedient? Did it remove immediate and possibly future problems, even as it created some moral quandaries? Sure; on balance I think fewer in-game problems were created by the killing than would have been created with leaving the helpless character alive.
Now please note that the preceeding paragraph can be applied to both the CdG that Technik4 performed (while party-mates were attempting to recussitate Anea), and the CdG that Mossimo performed on Technik4. There is absolutely no in-game reason why the reaction to the two different killings should be different: Either they were both reasonably expedient methods to remove a potential future problem, or they were both low-down dirty deeds. In the first case, you've no cause to be upset with Mossimo, since he merely applied the same reasoning as you did to the situation. In the second, both of you should be chastised; but while it may sound grade-schoolish: you did it first. If objectivity is what you're after, then you should feel your act is
at least as bad as Mossimo's, if not moreso because yours was the first: you should feel the same way because they're fundamentally the same act.
Now: introduce metagaming.
Technik4 is a PC, Anea is an NPC. In-game the acts are the exact same, so any conclusion stating Mossimo is more at fault than Technik4 must come from this difference. As players, you need to work together, so acts that may warrant death in an NPC may merely warrant a stern talking-to in a PC. OK. But take that principle to heart: you need to work together. This principle that you may hold up as a reason why Mossimo ought not to have killed you is the exact principle broken when you and the DM conspired to have a secret agenda for your character. There wasn't a unified front of players for Mossimo to betray because you, encouraged by the DM, had already destroyed that.
I think the DM here is the only person who gets out of this with a pass: Mossimo is now a PKer. You're a player who took his dice and went home when his methods were turned against him. The DM, who you say encouraged you to have a secret agenda, also ought to share in the burden for this. Hey, he may have just been trying to make something cool. He may have thought the idea of a secret-agent man in the party who would reveal himself and steer the party towards his agenda would be a great plot-twist. But for all his intentions, he invited discord into his group, and this situation is the fruit of his unadvisable choices.
So:
DM's: Don't divide your party. They'll do that enough without your encouragement.
Players: Don't have secret agendas, or at very least, don't seemingly betray the party in accordance to those agendas in the heat of battle.
Abide by those principles, and your PC won't have to worry about getting CdG'd by a fellow party-member.