Rancid means sour, rotten, and nasty and refers most specifically to the sharp bad smell of decomposing oils or fats. "Rancid" is a great name for a post-punk band, but a lousy way to describe the sauce on your eggs Benedict.
If you've never had the pleasure of smelling something rancid, life has quite a wallop in store for you. As meat, fat, or oil decomposes the chemical change creates a sharp wretched stink that will cause you to recoil. It's just foul. The powerful horrendous fragrance of rancid butter, therefore, makes the word wonderfully potent to describe all sorts of things — from a corrupt dictatorship to a party that's gone on a few hours too long.