A "bad apple" is someone who might not be in trouble with the law (as is the case for "a bad egg"), but is a person who has a bad attitude (often about work), who is a negative influence in a team, and who is generally not pleasant to work with or be around. Ex.: "That guy we just hired? I think he's a bad apple. He snaps at co-workers; he's becoming a real distraction." The new employee is creating negativity around him.