The HK police force was riddled with corruption in the past (1940s to 1970s), but then (after the creation in 1974 of the Independent Commission Against Corruption) turned around and became highly professional, trusted, and respected, I'd say ("Asia's finest").

However, with the 2014 umbrella movement, they were necessarily pitted against the pro-democracy protesters, having to do the government's bidding, and things got polarised. Polarisation got worse with the 2019 (until now) Anti-Extradition Law protests: restaurants etc. declared themselves "yellow" (pro democracy, pro protesters) or "blue" (pro police, pro Beijing). [1]

And, yeah, things got so heated then that the cohesion slipped away and, I think, the HK police did more than "what they had to" while pushing back against the protesters, particularly after Chris Tang's appointment as the Commissioner of Police in November 2019.

Police brutality, excessive force, refusal to show identification (as required by law) [2], maybe even falsification of evidence have severely deteriorated trust.

Now, with the total crackdown by Beijing, using the courts and the police as their tool, things are only getting worse [3]. :-/

[1] https://www.straitstimes.com/asia/east-asia/yellow-or-blue-i...

[2] https://hongkongfp.com/2019/06/21/hong-kong-activists-compla...

[3] https://hongkongfp.com/2021/01/04/explainer-how-the-national...