Stealth kills,
Killing someone stealthily, is unrealistic, with a knife, even with a silenced gun, because all guns make noise, silenced or not. Anyway back to fantasy, Slitting a throat the man could scream, or make a horrible gargling nise as he chokes on his blood, snapping a neck, the man could scream to, if you don\'t do it properly, it makes a big snapping noise.
The best stealth method, is with a dagger from behind, and jam it on a *I posted on page 6, never completed* 45 degree angle from the bottom base of the head, the object of puncture being the medula oblongata, or cerebrum, make sure to cover his mouth too! And try for in one smooth motion!!