a bad/rotten apple

a bad apple

A person whose own words or actions negatively impacts an entire group of people. Taken from the proverb "a bad apple spoils the bunch." Jeremy is really a bad apple. After five minutes with my usually well-behaved kids, they're all acting out.
See also: apple, bad

a rotten apple

A person whose own words or actions negatively impacts an entire group of people. Taken from the proverb "a rotten apple spoils the bunch." Before you accuse the entire department of wrongdoing, you should try to find the rotten apple that initially caused the problem.
See also: apple, rotten
Farlex Dictionary of Idioms.

a bad apple

or

a rotten apple

COMMON If someone is a bad apple or a rotten apple, they are very dishonest or unpleasant, and they have a bad influence on the people around them. It's an opportunity for them to make clear that they are not going to tolerate a bad apple in the United States Senate. In any profession, there's always the rotten apple, isn't there. Note: If a rotten apple is stored with good apples, it causes the good ones to rot.
See also: apple, bad
Collins COBUILD Idioms Dictionary, 3rd ed.

a rotten (or bad) apple

a bad person in a group, typically one whose behaviour is likely to have a corrupting influence on the rest. informal
See also: apple, rotten
Farlex Partner Idioms Dictionary

a bad/rotten ˈapple

one bad person who has a bad effect on others in a group: In response to the allegations of mass corruption within the team, a former player said today, ‘There may be the odd rotten apple in the pack, but the majority are clean and honest.’
See also: apple, bad, rotten
Farlex Partner Idioms Dictionary
See also:
  • a rotten apple spoils the (whole) barrel
  • a rotten apple spoils the (whole) bunch
  • a rotten apple spoils the (whole) bushel
  • bushel
  • a bad apple spoils the (whole) barrel
  • teach a man to fish
  • village
  • it takes a village
  • a man is judged by the company he keeps
  • a man is known by the company he keeps