frost
Executioner
Here's another possibility: suspension trauma. If I read this paper correctly: ordinarily, when a person's upright, the contraction of muscles in the legs pushes blood back up into the body. If the legs are hanging so that contraction doesn't take place, blood pools in the legs, and eventually the heart doesn't get enough to maintain circulation. Under this explanation, death by crucifixion would occur when the subject is too exhausted to push downward with his legs any more.