When any stat goes to zero, a monster (and PC) should be helpless. Helpless means:
Unable to move, attack, or take any actions. Helpless characters are automatically critically hit.
Anything less than this is poor design.
PS - while you're at it, take off the ridiculously high amounts of HP arbitrarily added on to monsters. Giants with 5000+ hit points are poor and lazy design meant to give the illusion of a challenge.