I'll agree that from the perspective of liberal historians, that yes slavery is what made it a "Good War".
But for me, now that I'm far removed from the influence of liberal history textbooks, I can safely say that the Civil War was a necessary war to preserve the Union, plain and simple. The abolition of slavery was a wonderful side effect. Slavery would have been abolished in due course anyway. The Civil War forced the issue by first forcing the issue of whether a state had the right to secede from the Union. Lincoln settled that question and no state has dared to try again since. If they try, they'll have to deal with far worse than Sherman.