Complex sentence: Milius based the character of Willard and some of Kurtz's on a friend of his, Fred Rexer, who had experienced, first-hand, the scene related by Marlon Brando's character wherein the arms of villagers are hacked off by the Viet Cong.
Split into simpler sentences: Milius based the character of Willard and some of Kurtz's on a friend of his, Fred Rexer. Rexer claimed to have experienced, first-hand, the scene related by Brando's character wherein the arms of villagers are hacked off by the Viet Cong.