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:

1. The police are an institution of oppression.
2. Everyone who joins the police does so voluntarily.
3. All police officers voluntarily signed up to participate in an institution of oppression.
4. 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.