a bad egg:
(plural: ‘bad eggs‘; the phrase opposite in meaning is ‘a good egg’)

a bad egg, threatening to kill
a worthless, dishonest person;
a wrong-doer; a person who does or tries to do harm to others;
a bad person in a group or team,

Nick Griffin, British National Party leader, accused of being a ‘bad egg’ by the Labour Party, England
Just because he’s been an irresponsible youngster just for once, everybody has started calling Andrew a ‘bag egg’.
Mary knew her boyfriend was a bad egg but she was could not break up with him because she loved so much.
It’s a fact that every family has a bad egg who makes the rest of the family lose their face before their relatives, but the Martins are such a big family that you need extra fingers to count all the bad eggs!
Every political party has at least one bad egg who spoils the reputation of the party, and the party chief and the other members have a tough time handling those bad eggs.
