As a gm, IF I edit it is adding 3 hp to NPCs, or +1 to attack. The encounters are generally not tough enough. This makes effectively 1 attack more getting through. Also, if the PCs are having a tough time spread out the damage. Everyone should get attacked at least once in every scenario. Most of the scenarios I have played the NPCs do not fight to the death. I think that if the encounters are too tough that is a result of bad GMing
I agree that the plot leaves something to be desired, but I think that most of this is the nature of the demon for non-modules. In Cincinnati, the events hop all over the scenarios, society members are not always current.
For the record, Crits are the only thing that should 1 hit kill, and NPCs don't have to finish off an unconscious PC. 1 hit drops are the name of the game. If they have a 1d8+8 and you think you are going to have to worry about a TPK take 2 off attack, it does wonders for letting tanks survive to run away. If they don't run when they should KILL THEM