In the practice of seppuku the one who decapitates the one committing ritual suicide

A 'second' in ancient Japanese culture. This person would act on behalf of the Shogunate. This would be an executioner who decapitates men after they have performed ritualistic suicide (Seppuko).
An example of this is the character Ogami Itto in the film series 'Lone Wolf and Cub'. He was chief executioner and would behead disgraced samurai with one fell swoop of his dotanuki.