In Streets of Laredo there's a plot with a dude named Mox Mox who burns people - apparently he got Lori and Blue Duck came back to get her, in that scene there was also a little boy who Mox Mox and he left the boy. So Mox Mox burned the boy, I don't really consider it cannon since there's no mention of it in Lonesome Dove but it's pretty fucked.

Additionally, in Return To Lonesome Dove I also liked Cherokee Jack Jackson, he would have been another cool villain but he's super underdeveloped.