General Sherman did nothing wrong, the south seceded from the Union before a vote was placed in US federal congress. Then when the CSA declared independence they fired on US military bases(which if the CSA is independent like they claimed) it was an act of aggression and war, a war the south started, a war the south supported, and a war the south lost.
The reason why reconstruction failed is a southern nationalist killed Lincoln and with that any hope of a very forgiving reconstruction for the south. Honestly, the south shouldve been under martial law longer.