The full saying is “a few bad apples will spoil the bunch.” Because if you toss a rotting apple into a barrel of good apples, the off-gassing from the rotting apple will quickly rot the good ones. You’ll quickly end up with an entire barrel of bad apples, due to adding one bad apple to the barrel.
Even “good” cops will quickly turn rotten, because they’ll be forced into covering for the “bad” cops. Because if they don’t cover for the bad cops, they get forced out.
Thus, all cops are bastards. Because if someone is a cop, they’re either a bad cop or covering for bad cops (and are thus, a bad cop themselves.)