I generally agree with this on an abstract level though I have generally found it best to assess police officers and/or claims of police brutality on an individual or situational basis.
It’s possible for a cop to become an ex-cop, and for an ex-cop to criticize many cop behaviors or functions. People have all kinds of reasons for becoming police officers. Some really do think they will have the chance to help others and fight bad people. Some want to drive fast and carry guns. Some really believe in the “law and order” ideology. Some view it as an opportunity to participate in crime and get away with it. For most, it’s just a job.
There were Vietnam and Iraq War vets who later joined antiwar protests, and there are former cops who participate in anti-police brutality protests.