Does it merely make them look bad? A bad cop is a more dangerous criminal than most of the people the cops are there to deal with. If the 'good cops' aren't enthusiastically hunting them down, I'd say that they are ineffectual at best and complicit at worst, not merely sullied by unfortunate proximity.
Very good point, but I suppose "good" and "bad" is a spectrum. The problem is, if a cop with the best intentions knows about bad/illegal behavior of another cop, what are they going to do? If they report them (at least if they're not the bad cop's superior), then the bad cop will still not be punished, but the otherwise good cop will be harassed and hazed until they have no choice but to quit or be fired (or in the case previously mentioned be thrown in a mental institution). So, would it do more "good" in the world to get shoved out the door and not effect change, or to continue doing what you can to bust the criminals that you can? I suppose the answer to that depends on how idealistic you are, but the real world tends not to be boolean.