The North invaded the South to "preserve the union", and only emancipated the slaves as a "lever" toward that end.[0] The South fought back because it was invaded by a foreign power--one which sought subjugate the South and force its inclusion in the American Empire, contrary to the will of the southern people. This is simply a matter of fact. The men who fought off the invader will always be Southern heroes, and rightfully so. The pretense that it is immoral to defend a revolution while also engaging in slavery is, coming from the land which claims Washington and Jefferson as its greatest heroes, so utterly hypocritical that it's hard to consider it to be a good faith argument. Because it's not. It's moral blackmail.
[0]Abe admitted as much in a letter: "My enemies say I am now carrying on this war for the sole purpose of abolition. It is & will be carried on so long as I am President for the sole purpose of restoring the Union. But no human power can subdue this rebellion without using the Emancipation lever as I have done. Freedom has given us the control of 200,000 able bodied men, born & raised on southern soil. It will give us more yet... My enemies condemn my emancipation policy. Let them prove by the history of this war, that we can restore the Union without it."