NPCs have different personality traits, some might fight and others will try and run, they have predetermined Fight or Flight mechanics.

No idea how the game will handle it, but a unit in that position would probably neither flee nor fight to the death. It would surrender.

"Ground on which we can only be saved from destruction by fighting without delay, is desperate ground." "On desperate ground, fight."

"Throw your soldiers into positions whence there is no escape, and they will prefer death to flight. If they will face death, there is nothing they may not achieve. Officers and men alike will put forth their uttermost strength. Soldiers when in desperate straits lose the sense of fear. If there is no place of refuge, they will stand firm. If they are in hostile country, they will show a stubborn front. If there is no help for it, they will fight hard." - Sun-Tzu The Art of War