If an NPC is about to be killed by guards, do you lose your ghost score if you kill the guards? What about knocking them out?

Please refer to the sticky post
Notes on Stealth Detection, Chaos, and Trophies/Achievements

which states friendly NPC's seeing you be aggresive counts as a detection.

From my understanding you'll lose ghost if a hostile detects you, so don't be seen when taking out the guards and you should be fine. You could always save beforehand and then quickly finish up the mission and check to see if you still have ghost.