And that's not counting those executed by firing squads in the regime he helped establish. Yet leftist proudly wave his image and wear his shirts at their "protests".
As for the civil war, whether or not it was treason is arguable. Those state seceding from the Union inherently meant the citizens of those states were no longer US citizens, and thus it couldn't be treason.
The Union never even tried Jefferson Davis, the Confederate president, for treason because the Union wasn't even completely sure their war effort was legal. They weren't 100% certain they could convict him, and if he was acquitted, that would publicly declare that secession was legal, and would mean the Union had broken the law. Then it would basically change the war from a civil war to a war of conquest, and the southern states would just end up seceding fully legally.
That's the main reason those statues were put up, because they were heroes to the Confederate states, who gave their lives patriotically, even when some of them didn't fully believe in the cause those states were fighting for. They weren't fighting to keep slaves, they were just (in their minds) fighting to remain free, and they didn't feel they were citizens of the US any longer.