Open the NPC's reference window, set the NPC's health at zero, and check-on the Corpse Persist function.
This will make the NPCs already dead when you enter the cell. (skips the death-animation that you would otherwise see if you don't check Corpse Persist)
edit: if you cannot set their health at zero, turn off Auto-Calculate first.
There is a caveat to this, which I ran into making a morgue for a mod. Yes, setting their health to zero will (obviously) cause them to die immediately, but they will still "randomly" fall depending on placement, distance from statics, etc. Fortunately, it remains the same once you have placed them.
You may not care precisely where your NPC's "die", but in my case, I really needed them to fall such that they lined up with the table, and I ended up having to use a script that called skipanim, and then manually rotated them once they were dead.