In an ideal world where we have an oracle telling us which cops are good and which are bad your comment makes sense. But the nature of corruption is to obfuscate its dealings. You can't just say "well we need independent oversight" then because there are so many institutional pivot points where cops can hide their abhorrent behavior before it gets to see the light of day.
In practical application ethics violations are knowable breaking of rules or demonstrable malicious intentions.
This is evidenced in how you think "ethics" can be defined by a wikipedia page.
The concept of ethics is very much different from its numerous instantiations.