Bad cops couldn’t exist if there were good cops.
That doesn't follow. That's like saying if there were any good people at a given organization, it wouldn't be able to hold bad people.
It actually does. If there were good cops they'd police and eject the bad cops.
You might have one or two here or there that got in, but they wouldn't last long.
Instead you see that even when bad cops are fired from their jobs, they just get hired somewhere else.
A few bad apples spoils the bunch.
Rotten apples produce gases that accelerates fruit ripening and turns everything else in the container rotten more quickly.