There are no “atheist ethics” as in because he’s an atheist precisely because there is no possible a priori basis for an ethical philosophy.
Calling it an “atheist ethic” because he’s atheist is like calling Hitler’s ethics a “German ethic” for being German or a “Christian” ethic because he supposedly was formally a Catholic as a youth, or a “Jewish” ethic because he was a relative (son or cousin) of a Jew (according to three different Jews I’ve known, two Orthodox and one Conservative).
The non-aggression principle is logically what mathematicians and logicians would call a “corollary” to the Golden Rule. The Golden Rule says Do unto others as you would have them do unto you. Of course that ALWAYS includes the non-aggression principle: You yourself NEVER want your neighbor (or a stranger) to do to you harm.
There is a distinction between “the state” as institution and the actions of individual rulers. The distinction is that “the state” is an abstraction, wherein “the state” cannot do anything at all unless the “individuals” who enforce the state (enFORCE the state) do it, supposedly on behalf of the state. So the abstraction is ONLY the sum of the actions of the individuals who are “members” of “the state”.
War by “the state” is always murder because it is “the state” that does it by force, whether by conscription or by stealing the resources to pay for the havoc it causes. James told us plainly where wars come from, the is, the lusts within us.
Read the last verses of Matthew 17, where Jesus says the rulers of the earth that collect tribute are always corrupt by definition and the rest of us are “not free” because we are not the children of those rulers.
When Paul referred to the state authorities as servants of God he was referring to those who ACT like it. Caligula does not qualify. Nero does not qualify. Obama most certainly does not qualify, ungodly pagan he is. Hillary Clinton, who threw every symbol of Christianity out of the White House and replaced them with demons and gargoyle statues, also does not qualify. Ahab and Jezebel, Caiaphas, Hitler, ANY of them.
THE ONLY CIVIL LAW GOD EVER GAVE US, the laws of Moses, BANNED GOVERNMENT. The Judges were raised up by God to drive out the oppressor until Israel demanded their own brand of oppressor, a king. Go read what God thought about that.