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.
Farlex Dictionary of Idioms. © 2022 Farlex, Inc, all rights reserved.
a bad appleor
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.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012
a bad/rotten ˈappleone 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.’
Farlex Partner Idioms Dictionary © Farlex 2017