It would be interesting to know how many South Vietnamese were killed by the Communists after their victory in 1975--again, a civil war from one perspective (the North's)--but there was an unspoken agreement after the fall of South Vietnam not to learn how many were killed by the Communists. Tito killed many of his opponents after winning what was both a civil war and a war against foreign occupation.
Charles II had a few of those most responsible for his father's beheading executed. Later, when Bacon's Rebellion was put down in Virginia, he was upset at the governor of Virginia for killing more people in punishment for that than he had killed for killing his father.
The insurrection of the south was a rebellion of the southern states against the Article 3 constitutional responsibility to resolve controversies at the supreme court.
Even with Taney as chief justice, the slave power knew they had no case, and instead, chose to violate their Article 3 responsibility, and resorted to violent rebellion.
And Taney was not arrested by Lincoln. The only primary source is by one person’s testimony, and that testimony was in a foolish book issued long after the war. No copy of the alleged arrest warrant has ever been offered.
The southern perspective on the War of the Great Rebellion is not valid. It was found to be invalid by supreme court case ‘Texas v. White’.
Secession would be legal, if pursued by constitutional amendment, or by federal law, or by supreme court case. It could be de facto valid if the slave power had won their war, and managed to wrangle a peace treaty, signed by the President, and ratified by 2/3rds of the Senate, but none of those methods but the last were achieved by the slave power.