So I unlocked the Touch of Death perk, but I can't seem to get it to work. From what I read elsewhere, it should replace normal from-behind stealth kills, but when I do that, I still get the old animation, and trigger an alarm if I am seen doing it.

Anyone else have this problem?

Ah, found the answer:

The kill has to be triggered by pressing the action key (E), not the key bound specially to stealth kills (Y).