The concept doesn’t seem that far out to me and I know McCarthy used historical facts as basis for much of the book. The ex-soldier (who may not have even been a soldier to begin with) is named Captain White and he actively recruits young and violent men into his group in order to steal land and money from both Mexicans and Native Americans. Just wondering if there’s any specific historical figure (or figures) who may have committed similar crimes.
Blood Meridian was based largely on the history of the Glanton Gang, led by John Glanton. One of Glanton's gang, Samuel Chamberlain, wrote a memoir, My Confession: Recollections of a Rogue, which might have more information of interest for you.
I'm not an actual historian though, so this comment will probably be deleted in order to preserve the sanctity of this subreddit.
Here is a previous question on a similar topic: https://www.reddit.com/r/AskHistorians/comments/1vh6kh/historical_accuracy_of_mccarthys_blood_meridian/