How to make killing a foe fail a mission?

Is there any way to do this? I thought making them a required escort, then having them betray you, would do the trick, but this does not seem to be the case.

I basically want to give the player two options to complete the mission (defeating the boss, OR, destroying several objects within a time limit). And i want the mission debriefing text to reflect what the player did, hence wanting to use the "mission succeeded" and "Mission failed" dialogues. Then again, the mission failed dialogue would have to reflect both killing the AV OR running out of time, but let me worry about that.

Any ideas?

Check @The Cheshire Cat's The Beating Heart of Astoria: A Play in 5 Acts (Or Astoria in D Minor, I forget which) - the final mission used to have a "boss" that you can attack, and killing him fails the mission. If it still works in that mission, then it's possible. If not, then it's no longer possible (I imagine).

IIRC, the mob in that mission is actually an ally that just stands there while you beat it to death. Probably not what's needed here. What is needed is, to the best of my knowledge, not available.

It was a required escort set to Rogue alignment, with no guards. It immediately became hostile when "rescued."

Can you make an escort betray as soon as they're rescued? Or do you have to complete the escort objective first? Or perhaps you can chain another objective off the escort rescue, one that is easily completed such as the release of an unguarded captive, then have the escort's betrayal chain off that.

Well, I kinda want the player to "fail" if they beat the person up... and i want them to be auto-kicked from the mission.

The mission is also currently timed, could that be interfering with the fail script?

Also, i have them Alone and have them Betray "when rescued" so they're hostile the moment the mission starts. I can kill them, but the mission doesn't fail =(

[edit] Okay, got it to work, but there's 1 problem: She SK's down to level 50 (from 54). It's not going to want to make people avoid her if she's a level 50 EB. At this point I might just make her an optional boss without a fail condition attached. Ah well, her killing the character is enough of a fail condition =P Thanks for the responses, all!

Already done, but heaven forbid a player like me sees a line like that and takes it as a challenge instead of a warning =)

AKA Don't touch the treasure...

I once had an idea for an arc that was meant to encourage the player to run away from the main antagonist(s) rather than fight them, by making them super tough and using dialogue to make it clear that you weren't supposed to fight them.

I never made the arc because I realized that EVERYBODY would try to fight them anyway.