Soldiers are trained to fight and kill foreign enemies, though sometimes they're used to handle own citizens.
Police are trained to maintain law and order within a country's own border.
In 1960s, police in US were referred to by young people as pigs. There's a good reason for it.
They've often acted as agents for the protection of the establishment in many countries.
In many cases they use unnecessary, brutal and excessive force against innoncent and defenseless civilians.
Many take bribes, use extortion to make monetary gains, engage in illegal activities, intimidate the public, and are trigger happy.
In such cases they're simply licensed and armed gangsters in uniform, acting in their own interest and the interest of the government and the establishment, and wouldn't it be appropriate then to call them pigs or even domestic terrorists?