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.
A remark you make when you are leaving on the purpose of creating a stronger impact
The letter he left before leaving was his parting shot.