Treason, slavery, and white supremacy.
This about sums it up. I would have been tempted myself to try Robert E. Lee for treason (a military officer sworn to uphold and defend the US Constitution, who resigned to join the enemy in a treasonous cause, rather than do his duty and lead the Union troops to a much swifter victory). What that man did was a disgrace, and cost
a lot of lives. And it never bothered his conscience either. I might add that if the South had managed to succeed, it may well have taken over most of the Caribbean and Mexico, and convert them into slave hell holes. The rebel South was a cancer that needed to be exorcised, and forever interred.