And yet the entire idea of a sleeping man's throat being slit is somehow preposterous to you?
The idea isn't preposterous. But this conflict is predicated upon the narrative ruleset of this game system, where maneuvers and Aspects are there to establish plot potential, not merely a simulation of reality. The resolution mechanism of this game system relies on clear understandings of what is at stake in each conflict.
PCs and NPCs ideally enter into each conflict with a goal: NPCs going for "tranq and kidnap the PCs" have to resolve that in a conflict with the
PCs players who decide their goal is "don't get tranqed and kidnapped." Even if the
PCs players lose, that adds another plot element to the game: now they have to find out who kidnapped them, why, and how to escape. Heck, some players can simply be encouraged to Concede to capture because it may get them closer to their enemy and an opportunity to really foul his machinations.
If the NPCs are instead an assassin squad intending to "subdue and murder the PCs" then the stakes are obviously higher and the
PCs players are going to do whatever is in their power to avoid that fate. And getting killed in one's sleep is a fairly non-heroic way to go with you game. So the
PCs players are going to want chances to "shake it off" rather than get autokilled, and if they are out of FP, the rules give them the opportunity to Concede to a lesser effect, like Left For Dead.
The game can be brutal and fatal if played the right way, but the rules are designed to keep the story going, not to ignominiously snuff the PCs because they opened the wrong door. We're creating a story, not a dungeon crawl. And if the
PCs players agree that the best thing for the story IS that their character snuff it, then they don't take the Concession. Problem solved.