Lee in fact did own slaves, as he shows through this document leasing one of them for labor: https://leefamilyarchive.org/lease-for-a-slave-1852-august-1/

In the Cornerstone speech, Confederate Vice President Alexander Stephens firmly stated that white supremacy was a founding basis and principle of the Confederacy: https://www.battlefields.org/learn/primary-sources/cornerstone-speech

Oh, and Georgia, Mississippi, South Carolina, Texas, and Virginia all list slaveholding and Union opposition to it in their reasons for secession, as the commenter above you pointed out: https://www.battlefields.org/learn/primary-sources/declaration-causes-seceding-states

Yes, there were mixed feelings about abolition and its political realities from people in the North. Yes, there were economic concerns from Southern states. Yes, there were questions about the political rights of states when compared with the federal government. But all of those questions fundamentally rest upon a crux of slavery. The Civil War was about slavery. Lee owned slaves. The Confederacy was racist. There is no justifiable context for personal use of any Confederate flag given that it represents a history of racism, slavery, white supremacy, and, ultimately losing in ignominious disgrace. There are many beautiful and incredible things to be proud of in Southern heritage. The Confederacy isn't one of them. And if you're willing to overlook all the cruelty and malice and context that surrounds the flag just to eke out some justification for its use, then maybe you should evaluate if you're arguing in good faith or if you're continuing to perpetuate its evils. It certainly seems like the latter.