It's not the number of people killed in the American Civil War, it's the percentage of the population killed.
Twenty five percent of all military aged men in the South were killed. Another twenty five percent of all military aged men in the South were maimed.
No country anywhere on the planet would see this percentage of casualties until certain areas, and only a relatively few, experienced such huge percentages during WWII. Heck, the world as a whole, only lost like what, two percent of it's population during WWII? And that was the most epic overall war humanity has ever seen.
You are forgetting the many small tribes which were defeated so badly that their tribal organization ended and the the survivors, if any, became assimilated into different peoples, thus ending their ethnic groups. For example, several nations that were ended by the Iroquois during the 17th century Beaver Wars, or the Yahi in California.
And among modern nations that had international diplomatic recognition several small nations involved in World War One and World War Two suffered civilian fatality rates equal or greater than the South's military fatality rate. And the winner, or rather loser, in this doleful competition is Paraguay which suffered the deaths of at lest 50 percent of its pre-war total population during 1864-1870.
I agree on the tremendous losses the South suffered ,it must have afflicted the South spirit grievously
as for some record on men losses ,
Paraguay war of the triple alliance was quite devastating with massive losses of men