Oversimplification of the Civil War and Lee. Yes, it is absolutely true and correct that slavery was the major underlying factor because it was the basis for much of the South's economy. However, not everyone who fought for the North opposed slavery and not everyone who fought for the South favored slavery. Many on either side saw it as being fought over a more fundamental principle of States Rights vs those of the Federal Government even though the Right of a State to allow slavery was the major item underlying States Rights concerns. States Rights believers held that since States voluntarily ratified the US Constitution and joined the nation that they should be allowed to voluntarily leave it. A considerable number of people back then also often felt more personal loyalty to their home State than they did to the United States as a nation. Lee was one of those who was seemingly more driven by not wanting to fight against his home State of Virginia than by a strong desire to preserve slavery. Union General George Thomas ( "the rock of Chickamuga") was a Virginian, who unlike Lee fought for the North. He appears to have been ambivalent about the institution of slavery having owned slaves himself at one time pre-war and helping some of his former slaves towards independence immediately after the war. As one might expect some other Union generals were suspicious of him because he was from Virginia.
In addition, it should be realized that the Emancipation Proclamation was made by Lincoln just as much to be a weapon to hurt the South's economy to help the end the war faster as it was made for the morally right and noble motive of ending slavery.
I highly recommend that you read the primary sources. Find out in their own words what slave states said about why they were seceding.
Georgia: SECOND SENTENCE "For the last ten years we have had numerous and serious causes of complaint against our non-slave-holding confederate States with reference to the subject of African slavery."
Mississippi: SECOND SENTENCE "Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world."
Texas: "(Texas) was received as a commonwealth holding, maintaining and protecting the institution known as negro slavery-- the servitude of the African to the white race within her limits-- a relation that had existed from the first settlement of her wilderness by the white race, and which her people intended should exist in all future time."
South Carolina: "We affirm that these ends for which this Government was instituted have been defeated, and the Government itself has been made destructive of them by the action of the non-slaveholding States. Those States have assume the right of deciding upon the propriety of our domestic institutions; and have denied the rights of property established in fifteen of the States and recognized by the Constitution; they have denounced as sinful the institution of slavery; they have permitted open establishment among them of societies, whose avowed object is to disturb the peace and to eloign the property of the citizens of other States. They have encouraged and assisted thousands of our slaves to leave their homes; and those who remain, have been incited by emissaries, books and pictures to servile insurrection."
Virginia: "The people of Virginia, in their ratification of the Constitution of the United States of America, adopted by them in Convention on the twenty-fifth day of June, in the year of our Lord one thousand seven hundred and eighty-eight, having declared that the powers granted under the said Constitution were derived from the people of the United States, and might be resumed whensoever the same should be perverted to their injury and oppression; and the Federal Government, having perverted said powers,
not only to the injury of the people of Virginia, but to the oppression of the Southern Slaveholding States." (emphasis in original).
Please, peruse the articles of secession. Read the personal correspondence between Southerners. Read the first-hand accounts of how the Confederacy treated free blacks - some soldiers - from the Union. Arguing that the Civil War was about something other than slavery is like arguing that that time of possession is the point of a football game.
