Imagine that, during Sherman's campaign while in the mountains going through Chattanooga and into Georgia, that for long periods of time his soldiers would be marching entirely up-hill, and thus having to try to fight uphill whenever there was Confederate skirmish
The presence of just a few confederate soldiers in an area as union platoons marched through would be a completely terrifying experience for the union soldiers - as confederate soldiers using guerilla tactics would send a lone bullet perhaps once every 5 minutes or so from an unknown direction, killing a soldier of the platoon, one at a time - with the platoon being helpless and without any means of fighting against it. After days of being in constant fear of sniper fire around every corner, with the dizzying and disorienting mountain terrain, you can imagine that everyone from the soldiers to the officers to the generals was completely gripped with fear, and were thinking of their entrance to the region as a terrible terrible mistake -- due to the terrain, no area felt as if it were cleared of enemy soldiers as they moved through them; they weren't pushing any line of occupation forward as they travelled. Strategically, they were deep in enemy territory surrounded on all sides.
When they eventually came out at the foothills of the mountains before Atlanta, they were so terrified of their enemy, and so deep behind enemy lines, that the idea of simply passing through towns when they came across them was viewed to be the strategic equivalent of ringing the doorbell to let the enemy army know you're there, walking right past them, and having a big sign on your back saying "shoot me". Not to say that the razing of towns in Georgia was justified, but a paranoid Union army that had been subjected to guerilla tactics throughout their journey through the mountains would be willing to do whatever they could to avoid any more skirmishes.
Hence, Sherman's march to the sea, although terrible, was likely not done out of any sort of lust for killing or anything of the sort, but rather done out of desperation.