The issue is that the good cops keep their mouth shut because police organizations rule through fear and intimidation. It's like a cult--either you're with us or against us. So, it makes the good cops go along with the corruption.
Yep - they sometimes fear the bad cops won't have their back, will let them take a bullet because they cannot trust them, etc.
Cops often see 1 cop lie about something, get excessive, or take something - and they just STFU.
If my buddy slices a hooker's throat, feeds her to the pigs, and sprinkles her teeth off Niagra Falls... and all I do is simple ride along, cover him while he's slashing, wash up after the pigs, and sign off on the missing persons report... well, that kinda makes me complicit in the hooker murder AND a piece of shit too. Same applies to cops when they look the other way.