I have the perk "Assassin's Blade" (Sneak attacks with daggers now do a total of fifteen times normal damage), but I don't know if it's required for these encounters. I get them almost every time I perform a sneak attack with a dagger on a human target. Just if there doing idle stuff like sitting on a chair, my character kills them with a normal dagger sneak attack.
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I think you have to use a power attack and a directional button... but I find it to be mostly random.