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Phrase(s): the villain of the piece
Fig. someone or something that is responsible for something bad or wrong. (Fig. on the villainous role in a drama.) • I wonder who told the newspapers about the local scandal. I discovered that Jane was the villain of the piece. • We couldn’t think who had stolen the meat. The dog next door turned out to be the villain of the piece.
McGrawhill's American Idioms And Phrasal Verbs