Is that they spoil the entire bunch. And the barrel they're stored in.

I work with an ex cop. He was a watch commander, but when he had to retire due to heart problems. In our discussion about bad cops he told me, "Everyone knew who they were, all you could do was stay away from them" He also told me that police have no compassion for people, because they are cops and aren't supposed to, which I found to be a bit disturbing.

If you think of it that way, or even "one bad apple spoils the bunch" it would fit. A few bad policeman, or soliders, or apples will lead to the corrupiton of the entire organization or barrel of apples.

Used that way, it reveals a truth that corruption is contageous.

Now, it is used to say that there are a few bad folk but everything is fine, and that is false..