I have been able to get knife kills in one hit by aiming at the head regardless of direction they face.
Yeah, I guess I need to quote what I said earlier.
Insurgency 2 also isn't one shot kill with a knife.
I just had it happen to me when I tried to kill someone from behind. He just turned around and killed me. So I understand it isn't in this one too.
It's supposed to be, but melee in Ins2 can get a little wonky.
I just back stabbed a marksman who was completely oblivious (wearing heavy armor) and after the hit, he turned around and shot me, meaning it didn't 1 shot kill. Is this a bug? or does the heavy armor prevent it from 1 shotting?
The Heavy Armor does stop the one-shot.
In fact, Sandstorm's melee damage model works just like the bullet damage model. Torso hits are only effective against unarmored targets, but aiming for the opponent's head is an instant kill regardless of what direction you hit him from. It's kind of like the Heroes and Generals melee system where headshots always instakill.
That being said, you can perform a melee charge if you melee attack while sprinting. From what I tested yesterday it seems to one-shot an enemy consistently regardless of where you aim or what armor your enemy has.