bad apples

Put a bad apple in a barrel, leave it there for a short period of time, and observe that all the apples are now bad. Rotting apples release ethylene gas which is a ripening agent. Through the release of this gas, bad apples will "spoil the barrel" if they are not diligently weeded out and disposed of. Bad cops don't release ethylene gas. The mechanism that rot uses to spread through a police organization is called "The Blue Wall of Silence".
I love the "bad apple" metaphor because when you remember to say the whole thing it becomes clear that it is actually an argument against ignoring issues in an organization because only a few people are the "cause". If a police department "only has a few bad apples" then (according to the metaphor) those bad apples must be removed swiftly and decisively or the integrity of the entire department will be compromised.