villain of the piece, the

The person to blame for what is bad or wrong, as in I'm afraid the caterer is the villain of the piece—the food wasn't ready for hours. The piece in this term alludes to a play in which the villain is the character whose evildoing is important to the plot. [Mid-1800s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.