Unconcious (Save or Damage ends). Think Mean.

[ValhallaGH]

Just curious if any DMs out there allow a player to walk up to an NPC that is sleeping and just slit their throat for a instant kill? Seems to me if we are thinking realistically and that NPC has been knocked out or put to sleep by a spell, they aren't defending themselves and it could easily be done. Thoughts? Comments?

Hell no. Realism has no place in my games. It isn't fun, or balanced, and it forces players to make characters every two encounters, because they got stabbed by a sword, three times in ten seconds, and died.

As for "instant kill throat slitting" that's what coup de grace is there for. Coup de grace is the game mechanic for killing a helpless foe (or ally). It can be used by anyone on any valid target using any ability they have.
Now, I've been known to let a standard guard be a minion as long as the PCs have snuck up on him. This creates a very Splinter Cell vibe of "stealth good, combat less good" which really fit some of those scenes. But any sort of "instant kill" creates too much mental confusion for myself and my players when some of the critters can be instantly slain, while others can't.