Complex sentence: LeBoeuf proposes to team up with Cogburn, who knows the Choctaw terrain where Chaney is hiding, but Mattie refuses his offer because she wishes Chaney to be hanged in Arkansas for her father's murder, not in Texas for killing the senator.
Split into simpler sentences: LeBoeuf proposes to team up with Cogburn, who knows the Choctaw terrain where Chaney is hiding, but Mattie refuses his offer. Mattie wishes Chaney to be hanged in Arkansas for her father's murder, not in Texas for killing the senator.