>>72951711The act of consuming another's buttocks (Assumed to be their rectal entrance) is in turn an act of submission. It is inherently disgusting and an act of sadomasochism. Shiori, in telling the boss she just killed to eat her butt was asserting her dominance over an enemy she humiliated.
Or she just likes rimjobs, we will never truly know what goes on underneath her oreo hairdo.