The winners wrote the history books...
But as Birdwatcher pointed out, the Southern leadership wrote their documents of succession, and they made slavery the centerpiece of those documents; slavery was the centerpiece of the South's causes for succession; and the Southern leadership wrote their Constitution, and they made slavery the centerpiece of that Constitution. That's what the Southern leadership wrote.
With a million or more casualties in that war, it's certain that innocent people on both sides were killed...and it's certain that atrocities happened on both sides.
