What needed to be done? Why were 7 states trying to go it alone needing to be subdued by the Federal power? What threat did they pose to the remaining states?
In 1776, or as Lincoln put it, "Four Score and Seven Years ago..." this nation was founded on the principle that states had a right to self determination, and this was the point expressed in the Declaration of Independence that declared Independence to be a right given by God.
In other words, by our own foundation document, states had a right to leave if they chose to do so. By what right of law or principle did Lincoln have the authority to force states which joined consensually, to be forced to remain through coercion?
When I say ‘great’ I don’t mean great in a Jesus sense... I mean ‘great’ in the sense he completed a monumental task, which isn’t debatable.
So did Joseph Stalin, but that doesn't make him admirable. When the task you have accomplished is killing hundreds of thousands of people, and wrecking millions of lives, should the man who did it be admired?
Let me remind you, Lincoln's "Task" was not to free the slaves. He was trying to cut a deal to keep them in bondage. His expressed will was to keep the Southern States under the control of Washington DC.
Unknown to most people, the Southern states were producing the vast bulk of American trade with Europe, comprising about 73% of the total revenue production from Europe. They were therefore also paying 73% of the taxes, with the Northern states only contributing about 27% of the total federal revenues in 1860.
In other words, the Southern states were Washington DC's cash cow, and they didn't want to let their cash cow go. Lincoln's reasons for going to war had nothing to do with slavery, and everything to do with money and the control of Southern export production.