For fatalities to the head (beheading, skull crushed) the bloodthirsty attacker still needs to land a killing blow to the head. Conversely, for a fatality to the body (disembowled), the killing blow needs to connect with the body and not the head. The bloodthirsty trait sets the chance of a fatality occuring to 100%, regardless of weapon/skill, but does not change where the killing blow lands.
