The problem with beth's sneaking mechanics isn't one hitting stuff.
When you can approach a dude undetected you should be able to just off him by slitting his throat or stabbing him without regard for his HPs or armour or whatever.
The problem is that you can do so repeatedly:
Imagine that you're a bandit.
You have set yourself up a nice cavern hideout or whatever with a number of equally crude and unwashed comrades and make a living ambushing and stabbing people who look like they might be carrying valuables or something to drink but don't look like they might put up a fight.
Life is good.
Then, while you're chilling out with some other dude, drinking ale and swapping some crude jokes, the dude's throat suddenly splits wide open or maybe he sprouts an arrow from his eye. Now, it's not like getting sprayed with someone's hot blood or seeing a throat slit is a particularly shocking (or new) experience to you, but it could have been fucking you this time, besides, you kind of liked this guy (as long as he didn't want too much loot for himself). You pursue fleeing shadowy figure while alerting the others, but you lose it in the shadows.
What do you do?
a) scour the area, preferably in pairs, not stopping until you find the killer or make sure he disappeared because THE GUY IS STILL AROUND AND CAN MURDER YOU AT ANY TIME
b) resume chilling out with another dude and drink some more ale, repeat until all of you get killed horribly and shadowy figure makes off with all your stuff
If killing is detected or friendly corpse is found the AI should not stop their search. And actively searching for you should make it almost impossible for you to one-hit someone again, doubly so without immediate counter from target's friends.
Then again, I don't recall a single RPG where stealth wouldn't be broken.
It's mostly implemented as hide-in-plain sight too.