"Twin-sister of Religion, Selfishness! / Rival in crime and falsehood, ____ all / The wanton horrors of her bloody play” (Shelley) (5)
I believe the answer is:
aping
(Other definitions for aping that I've seen before include "Imitation" , "Making copy of" , "Mimicking like monkey" , "Taking off" , "Sending up" .)