I've tried Googling this for a couple of minutes, but the results didn't even come close to what I was looking for. When you grab somebody in MGSV, you can slit their throat (or stab them to death or w/e; I haven't paid that much attention yet). Is it possible to do this if they're already knocked out? I don't want sleeping enemies waking up, I don't want to shoot them with a non-silenced assault rifle, and, aside from throwing them off of guard towers and stuff, I don't see any other way to kill them.
The only way to slit someone's throat, is during a CQC hold. To CQC grab and hold someone, that person has to be conscious and awake.
So all you need to do, is wake them up, grab them as they stand up, then slit their throat before they choke again.
To wake them up, get close to the body and press the attack button1. Big Boss will kick the soon-to-be-dead body, which will instantly wake them up2.
It works every time, and can also be used on prisoners to see if they're still alive after being shot at while carrying them.
Alternatively to kicking them, you can wake them up by looting their bodies. Typically, you will be able to retrieve 3 items3 from enemies before they wake up.
1: It's the same button you press, when you want to CQC grab an enemy.
2: Enemies do this, when they find an unconscious ally.
3: Mostly ammo.