Think about it this way. As a cop, your job isn't to help the community or to keep people safe or any of that happy wholesome crap. Your job is to enforce the law. That's it. It doesn't matter if the law is unfair or unethical, it's your job to enforce it. Sure, maybe some people become cops without fully understanding this, but on some level, they must know.

The laws are made by politicians, and I'm certain that no matter your political beliefs, you can agree that most of them are crooked. Ergo, everyone who signs up to enforce their laws is a bastard. If somebody truly wishes to serve the community and save people, they become firefighters instead. It's almost the same skillset and if you're willing to become a cop, you shouldn't be afraid of the danger either.

I elaborated a lot more on this in a different comment.

The main thing is it is about the participation in the system that makes all cops bastards. In my opinion, the good thing about this slogan is it starts discussions and debates about the system itself and how even "good cops" contribute to it when cops do something bad.