I feel like the rules of 5e (I'm not familiar with other editions) are really lacking in department of stealth opportunities. I'm mostly talking about one shot kills, but not necessarily. Examples:
- A PC manages to sneak behind a guard. Cinematically (as I understand it doesn't really work like that in real life) he would hit him on the head with a stone to make him unconscious, so the party can all walk over him to the next "checkpoint".
- PCs sneak up on a sleeping cult leader, and would like to put a dagger through his throat.
I really don't want to ban such actions from my games, they are cool, but rules don't really support them also. Sneaking up to an NPC takes at most a couple of Stealth checks, so in terms of gameplay it's really easy to do. I can't think of a good check to give players for putting a dagger in a sleeping human. It's really a trivial action. Hitting someone with a stone is a bit harder, but not that much, so the same concerns apply.
I don't think using "Attack" rules works - firstly, rolling attacks implies that you can either miss, or not make any damage through the armor, which is really not the case with a sleeping/unaware human. Secondly, if you roll attack, then you probably should roll for damage too, and even something like 20 damage (40 on a crit), which is really a lot for a single physical attack (something like 12 from rolling, 4 from STR, 4 from magic enhancements), is only enough to take out of action weaker characters like Thug (CR1/2, 32 HP). A Bandit Captain (CR2, 65 HP) will withstand such an attack somewhat easily. And, more realistically, you'll get something like 5 damage (stones and stakes are improvised weapons - 1d4, no proficiency, no magical damage on top), and that's barely enough to kill a commoner.
So, do you have suggestions on any good mechanics, that would make it possible? They have to both feel fair to the players (no misses/low damage on unmoving unprotected objects), and not make it a walk in the park (one stealth roll killing a Gladiator (CR5, 112 HP).