Only ones that would be included in warfare was the NCR guys.
Tricking a greedy mayor and then torturing and slaughtering at town is not warfare.
That is not defined as warfare.......... None of them deserved it but NCR, because NCR soilders is who they are fighting a war against.
It wasn't just about warfare, it was to show the rest of the Mojave what happens to people like the citizens of Nipton and Powder Gangers.
Hell, it's even nice enough of Legion to give the rest of the wasteland a heads up of things to come if you're a bad person.
Another thing was to show their power, that a small town as Nipton fell incredibly easy to the power of the Legion.
It's a tactic of bringing fear and terror to the rest of the people in the Mojave, and it's also meant as a way to demorilize NCR morale.
A third thing was to let some of them survive in order to tell people first hand of what happened in Nipton, the ones they left alive are basically walking advertisemants of Legion's brutality.
Next up is that the town was bad.
It was a greedy cesspool of evil dikes who would stab their own mother in the back for some cash, good riddance.
There were lots of good reasons to off Nipton for Legion, it wasn't done because they're sadistic bastards who like to make people suffer, it was done for the good of the Mojave, to stab NCR morale, to give the rest of the Mojave a heads up of things to come if you're a bad person.
So it wasn't just for psychological warfare but to introduce the Legion law of doing things to the people they are going to conquer.
In warfare though, no matter how evil one might consider it, killing "civilians" on the oppossing side
can be a good thing, you might not like it but even though they're not soldiers it's still warfare.