Furthermore, there was no requirement to stay in the union when the US was formed
You've never read the Articles of Confederation, have you?
Your argument is basically, "The South held slaves, the North were angels trying to swoop down and protect the helpless slaves from their Southern oppressors. Sherman's killing of civilians is perfectly OK in that context."
Nope. My argument is that it was a total war and Sherman destroyed targets of military value. He didn't directly kill civilians; he rendered some civilians homeless, which is a difference that is apparently lost on you. The wanton killing and aimless destruction that you're imagining is a figment of Southern imagination. Special Field Orders No. 120, emphasis mine:
IV. The army will forage liberally on the country during the march. To this end, each brigade commander will organize a good and sufficient foraging party, under the command of one or more discreet officers, who will gather, near the route traveled, corn or forage of any kind, meat of any kind, vegetables, corn-meal, or whatever is needed by the command, aiming at all times to keep in the wagons at least ten day's provisions for the command and three days' forage. Soldiers must not enter the dwellings of the inhabitants, or commit any trespass, but during a halt or a camp they may be permitted to gather turnips, apples, and other vegetables, and to drive in stock of their camp. To regular foraging parties must be instructed the gathering of provisions and forage at any distance from the road traveled.
V. To army corps commanders alone is entrusted the power to destroy mills, houses, cotton-gins, etc., and for them this general principle is laid down: In districts and neighborhoods where the army is unmolested no destruction of such property should be permitted; but should guerrillas or bushwhackers molest our march, or should the inhabitants burn bridges, obstruct roads, or otherwise manifest local hostility, then army commanders should order and enforce a devastation more or less relentless according to the measure of such hostility.
VI. As for horses, mules, wagons, etc, belonging to the inhabitants, the cavalry and artillery may appropriate freely and without limit, discriminating, however, between the rich, who are usually hostile, and the poor or industrious, usually neutral or friendly. Foraging parties may also take mules or horses to replace the jaded animals of their trains, or to serve as pack-mules for the regiments or brigades. In all foraging, of whatever kind, the parties engaged will refrain from abusive or threatening language, and may, where the officer in command thinks proper, give written certificates of the facts, but no receipts, and they will endeavor to leave with each family a reasonable portion for their maintenance....