I think you need to look at page 73 of the players manual....
KILLING BLOW
A person who is incapacitated by damage is defenseless. If it’s a human being
and you want to kill them outright, you must fail an EMP roll (roll one base
die only). If the roll succeeds, you simply cannot force yourself to commit
the deed. Even if the roll fails and you do kill the victim, you suffer 1 point
of stress – killing in cold blood is not easy. If you have the Killer specialty
(page 49) you can kill defenseless enemies without these negative effects.
Seems like if you fail the EMP roll you kill them out right but take 1 stress. Personally love this mechanic..it is a failure to to kill a helpless person
