Obviously spoken by somebody who has never stuck a real knife in a real person.
This game tries to be real. In real life, there is no such thing as a silent kill. There is always a fair amount of noise (and usually no instant death), easily noticeable to anybody in the same room and likely in the next room as well. Or a fair distance away outdoors, especially if there’s little ambient noise.
Stealth is for sneaking up on this one target, so you can deliver a lethal, surprise attack. The purpose for delivering a lethal, surprise attack is that it’s about the only way you can reasonably expect to come out of a knife fight unscathed. If you give the other guy any chance to fight back, you’ll almost certainly end up with a nasty cut or stab even if you win (and you might not win). You can’t block every attack and even those you block will likely cut your arm badly. So you always want to “stealth kill” your opponent even if there’s not another soul for miles around. It’s simple self-preservation.
“Stealth kills” are NOT for moving through a darkened building like the Angel of Death, silently slaughtering roomfuls of sleeping people. That’s pure hollywood and legend. You always make noise doing it, and this noise sounds exactly like what’s causing it: extreme violence. This is immediately recognizable to most folks due to millennia of evolution, and especially to those with any military experience, like guards, who have likely done it themselves often enough.
So, really and truly, if you do a “Stealth kill” in a room with anybody else in it, awake or asleep, you’r busted. There’s a good chance folks rather farther away will hear it, too. Think about how far away you can hear somebody drop a cardboard box across your house, or hear the kick of a soccer ball across a crowd of screaming fans. And once one guard hears the unmistakable sounds of violence, he’ll raise the alarm and the rest will come running.
That is what you should expect from a game that tries hard to be realistic. If you’re expecting something else, you’ve played too many fantasy games. Because it’s pure fantasy to expect anything resembling instant, silent kills.