And what do you thing the law is? It's a written agreement, because even if it doesn't give justice in every situation, it is better to peacefully agree on something instead of constantly fighting over everything. In other words a deescalation measurement.

A concept of a written law is very different from a rule by decrees and the will of the ruler. The difference is that deciding whether something is lawful is outside of the ruler or the enforcer. (i.e. separation of powers) This is what differentiates a liberal democracy from a dictatorship, but also 19th century and modern monarchies from absolutism. So no police is not modern men-at-arms.