“Who killed him and cut off his head?”
I am going to guess it was someone from the wife’s family. Loveless probably contacted someone for help and they contacted the wife’s family. Or, whoever helped in the escape was the one who wanted revenge.
This was a time when almost everyone butchered their own meat and met out their own justice. Odd, though, they dismembered him. That’s what leads me to believe it was a revenge killing. If it was just your average killing, they wouldn’t even bother burying him.
Movie, staring Clint Eastwood, The Outlaw Josey Wales (1976) delivers the line,- "Hell with {them} that fella[s]. Buzzards Gotta Eat, Same As The Worms!!"