I'm not justifying anything. And they would have new cops since then.
Even if you say "It takes 5 years for a good apple around a bad apple to turn bad" then you will always still have good apples so long as with time you are adding new cops not all of whom are bad.
I think you think you can't hate corruption in police without generalizing, and you can because your issue is with the institution and not all the individuals.
You would have to give up hating any given cop by virtue of them being a cop, but at least then your anger would be directed appropriately.
Throwing fresh apples on a spoiled vat doesn't make the spoiled vat any better.