One bad apple spoils the (whole) bunch proverb
Used in a situation in which only one person, thing, element, etc., ruins the entire group, situation, project, etc.
John's mistake affected the team's performance. One bad apple spoils the whole bunch.
That scandal destroyed all his reputation. One bad apple spoils the whole bunch.
This proverb alludes to the fact that a decaying apple can lead other apples around to rot as well.