Yep, the police serve the themselves. The same can be said about basically every other group of humans. Police have significant power and leeway because the service has provides a lot of value to others.

I like your martial metaphor.

Poor and marginalized communities usually want more police, not less. Not because the police come at no cost, but because the alternative is worse.

It is like hiring one knight errant to keep other knights/bandits at bay.

This is basically the origin of all governments and power structure.