Not all violence by the state is legal. In a properly functioning democracy, the state cannot carry out arbitrary violence with impunity, only that which is consistent with the powers granted to it by the constitution and laws written and passed in accordance with that constitution. That was the case in the US for long stretches of its existence.

But under an authoritarian regime, it doesn’t matter whether what the state does runs contrary to statute or constitution because there is no one who has both the will and the ability to enforce any restrictions against the state.