Sentence:  Thus man devotes his brother, and destroys; / And worse than all, and most to be deplored / As human nature's broadest, foulest <WSD>blot</WSD>, / Chains him, and tasks him, and exacts his sweat / With stripes, that mercy with a bleeding heart / Weeps when she sees inflicted on a beast. 
Meaning of the word marked with <WSD> tags:  A stain on someone's reputation or character; a disgrace.