I seem to be having a problem with getting this skill to work -- I have one level in Blood Thirsty and I've so far assassinated about five or six enemies in stealth, using my sword, yet I have not yet seen any prompt (a red screen flash and a CTRL+LMB UI popup as I have observed in a video) for maxed adrenaline allowing me to perform a fatality maneuver. Do I need more kills, or am I missing something here?

I should also mention that I have the skill Shadow Kill.

I didnt even know the game had anything called adrenaline before the end. There is no tutorial on it, no adrenaline bar and no indication that the game mechanic is even in the game.