He was a man who took up arms against his own nation.
Lee ought to have been grateful he wasn’t hung.
His nation was a nation that had been peacefully voted into existence by the constituent states, one of which he was a citizen of. And he understood that distinction. Prior to the 14th Amendment, there was no such thing as a “Citizen of the United States”. You were a citizen of the State that you were born or resided in.
Thus he no longer saw himself as related to the States which continued to operate under the 1787 Constitution.
So his “own nation” was not the one you assume. That nation staged a provocation at Ft. Sumter to create an excuse for an invasion. They were not there to abolish slavery nor even to secure the fortress. It was merely to force a group of sovereign states to repudiate their own vote - peacefully taken - and continue as vassal states of an emerging centralized superstate, which was contrary to the very provisions in the 1787 Constitution prohibiting that.
So no, in Lee’s view, he never “took up arms against his own nation”. The overall nation that he lived in simply changed. The former group recruited hundreds of thousands of immigrant aliens to help them do the dirty work of forcing them back into a shotgun marriage; those people had no understanding of the original compact, and thus could be used to abrogate it under the absurd legal interpretation that Lincoln applied.
Had Lincoln been a more reasonable person he might have been able to negotiate a better deal for everyone, but he wasn’t. His behavior in the Illinois legislature shows that.
But he wasn’t and his bizarre interpretation of the nature of the Constitutional compact led him only to force. As Lee said, he could not see remaining in a association that was not based on mutual consensus. Everyone in 1787 - like his father, the second greatest hero of the Revolution - would have agreed.
That group of people wrote these words earlier in 1776:
“When in the Course of human events, it becomes necessary for one people to dissolve the political bands
which have connected them with another, and to assume among the powers of the earth, the separate
and equal station to which the Laws of Nature and of Nature’s God entitle them, a decent respect to the
opinions of mankind requires that they should declare the causes which impel them to the separation”
If the colonies of 1776 had such rights, tell us all why 11 descendant states had no such right. They clearly believed they did, and voted in their legislatures as such.