User blog comment:Xilinoc/Ch. 565 (Mangapanda is run by lazy barnacles)/@comment-26420452-20140122160104/@comment-2220112-20140122221854

The part of their soul that they drew their power returns to Yhwach, not their entire soul. Yhwach intends for his soldiers to die so that he may continue to live. Yhwach may be close to immortal and that Yamamoto just couldn't kill him.