Complex sentence: Gordon going along with the plan because then-captain Gillian Loeb threatened to kill the boy as the only witness if he talked--he becomes a hitman in hopes of "avenging his parents again and again".
Split into simpler sentences: Gordon going along with the plan because then-captain Gillian Loeb threatened to kill the boy (as he the only witness) if he talked. Wrath would later became a contract killer in the hopes of "avenging his parents again and again".