Complex sentence: The Texans thought they were going against their word, because the Comanche chiefs did not return all of the white captives and figured they held back some of their white captives to guarantee their own safety.
Split into simpler sentences: The Texans thought they were going against their word, because the Comanche chiefs did not return all of the white captives. The Texicans figured they held back some of their white captives to guarantee their own safety.