Your argument is clean and logical, but I have this sneaking suspicion even well-meaning liberals aren’t ready to hear it, and won’t be for a long time. But, it’s as simple as:
- The police are an institution of oppression.
- Everyone who joins the police does so voluntarily.
- All police officers voluntarily signed up to participate in an institution of oppression.
- People who voluntarily participate in institutions of oppression cannot be good.
I would say the only exception to this would be someone who signs up to join the police force as a clandestine agent of The People, specifically to undermine and dismantle it from the inside, but you never really hear about people like that. The closest thing is ex-cops who woke up and now fight against police corruption from the outside.