Here’s the thing; he was tasked with winning. You win by killing the enemy and taking away his support base. News flash! That involves killing. Lots of killing.
If I recall correctly, they burned down buildings and crops but didn’t kill the people if nobody was shooting at them. (I’m sure there were exceptions as you can’t control the discipline and quality of that many people who are pressed into service. But did Sherman give orders to kill women and children and non-combatants? If he did, it is news to me.)
By his own admission he was.
He ordered fire into areas that were undoubtedly being used as military positions by the enemy - just like we always run into in the middle east. I think the treatment of Atlanta has been exaggerated.
There were no laws of war. There were no rules for guerrilla war in particular, and there was a lot of it in the Civil War.