(bad|rotten) apple

a person who causes trouble for others in a group and has a bad influence on them

The idiom "bad apple" finds its origin in the concept of a single spoiled or decayed apple that can affect the entire batch of apples due to its contagious nature. In a metaphorical sense, it is used to refer to an individual or element within a group that displays negative behavior, poor character, or corruption, which has the potential to influence and disrupt the entire group or organization.

Unfortunately, one bad apple in the team's leadership led to a decline in the group's overall morale and productivity.
The school prided itself on its excellent reputation, but one rotten apple among the teachers tarnished its image.
In any organization, it's essential to identify and address bad apples before their negative influence spreads.
The project's success was compromised by a single bad apple who consistently missed deadlines and created conflicts.
In a harmonious community, addressing the behavior of a rotten apple is crucial to maintain peace and cohesion.