It doesn’t matter if they’re wearing a helmet or not, but the killing blow has to strike the head for a beheading or for being smashed. Likewise, the killing blow has to strike the body in order for the victim to be gutted. The latter doesn’t stop undead from being resurrected, though. The Bloodthirsty trait just sets the chance of a fatality occuring to 100% given the correct bodypart receives the killing blow.
