The critique is a bit too broad to explain simply in a lemmy post.
I can try, but keepin mind that this will not be exhaustive:
The police’s job is to reinforce the current power structures and keep the people that currently are in power at the top. That has been their historical purpose, too. Dating back to the 1800s when they violently beat down strikes and workers’ protests. They are always “legitimized” by “the rule of law” without adressing how legitimate the law is. Speznaz, Gestapo and the Stasi all “upheld the rule of law”, but where highly immoral. The same goes for Frontex.
When the police acts immorally and/or breaks the law, the social structures most likely will prevent repercussions for those police officers. If you get beat up by police, other officers will pretty much always cover for their colleagues.
The image of the police’s job being to “protect and serve” is the result of active police propaganda (so-called “Copaganda”)