Complex sentence: In a final twist the murdered man's creditors successfully petition the governor to overturn Tom's death sentence, because now he is shown to be black he is a slave, and as such is rightfully their property.
Split into simpler sentences: In a final twist, the murdered man's creditors successfully petition the governor to have Tom's death sentence overturned. Now that he is shown to be black, he is a slave, and as such, is rightfully their property.