"But surely there must be some good cops!"
First of all, let's talk about what it would mean to be a "good cop." A cop's job -- in theory -- should be to protect civilians' safety, rights, and property, in that order of importance. (maybe we could argue in some instances which comes first between rights and safety, but we should be able to agree that both take precedence over property.) They are implicitly authorized to use force to accomplish this, though ideally they should always use the bare minimum amount of force required in any situation.
This should include protecting civilians from bad cops. In other words, if you have one cop who is abusive and 10 cops who know about it and don't stop that cop -- by force if necessary -- then you have 11 bad cops and zero good ones.
Now consider the George Floyd protests last summer. I can give you links to specific videos if you require them, but I think that's unnecessary, because we all saw them (or should have seen them). Video after video after video of one or two cops roughing up suspects while dozens or even hundreds of cops looked on and did nothing. If there are indeed "many" good cops, the odds that a gathering that size would -- by sheer chance -- contain none of them, are so low that they're not worth considering. A much simpler explanation is that "good cops," if they exist, are so few and far-between that those abusive cops could safely assume there were none present who would stop them.
And that assumption -- by all appearances -- tends to be correct.
Second, it has been repeatedly shown that cops who DO speak out against other cops are punished far more often than the cops they are speaking out against. For a cop to actually hold other cops accountable is career suicide; in other words, all of the would-be-good cops get filtered out, leaving only the bad ones.
Examples:
Third, let's talk statistics. The percentage of police who are domestic abusers is MASSIVELY higher than for the general population. 40% of households that contain at least one cop report domestic abuse incidents. And that's just the ones that are reported -- odds are it's a lot higher. Let me know if you want sources to confirm that figure.
Fourth, police are explicitly trained to, by default, think of the public as the enemy, and think of themselves as warriors against said enemy. Kind of odd for a group that's supposed to "serve and protect" the public, right?
Fifth, it is established precedent that
cops have no responsibility to protect the public, despite that being literally the entire purpose of having cops.
Sixth, our criminal justice system is designed in such a way that poor and working class people feel the brunt of it far more than rich people. Much of the most severe social harm is not criminalized, because it is generally perpetrated by rich people. For example, if you steal $100 from your employer, that is theft, a criminal offense, which can get you sent to jail. If your employer intentionally underpays you by $100, it is a civil offense; most of the time they will get away with it, and even if they don't, all they'll have to do is pay you back the $100; they will never see jail time for it. As enforcers of this system, the ACTUAL purpose that police serve in the real world is one of keeping the working class in line, not one of actually protecting civilians in general.
And that's all without even touching on statistics about racist policing practices. Or historic examples of explicit and deliberate racism by police (such as the fact that they originated as slave catchers, that even after slavery was made illegal cops deliberately went out to catch black people for minor offenses to be used as prison labor, or documented cases of police planting evidence, or documented cases of police deliberately fueling drug problems in low-income black and brown communities, or..... you get the idea)
In short, all cops are bad, because the institution of policing is bad, and anybody who would voluntarily work for it is complicit. Some start out wanting to do good, but in a fundamentally corrupt system, sooner or later you have to make a choice.