Lee was not defending state's rights to practise slavery, as you insist on saying. He was defending state's rights, period. Why can't you understand the distinction?
Slavery may have been reprehensible, but the fact that it was practised in the southern states did not make those states ineligible to enjoy their natural rights.
If slavery, ipso facto is "morally reprehensible" tell me why then is it not condemned outright in Holy Scripture?
Abolition of chattel slavery was just the convenient excuse for the subjugation of the South.
Slavery was a part of a culture and way of life which the industrial North needed to get rid of by any means necessary. If you will do some serious study, you will find many Southrons who acknowledged the evils around the institution. Slaveowners who mistreated their people, as Marse Robert always called his slaves, were the scum of the earth - a.k.a. white trash.
Many took care that their slaves were treated humanely, kept families intact and enabled the Africans the blessing of the Christian faith.
The abolitionists goal of making the South Northern was a post facto declaration of war. Back then, the rights of the states actually meant something to the citizens of the state.