Board Thread:Questions and Answers/@comment-24826159-20140416214818/@comment-8986886-20140416231219

If the Shinigami is killed by a Quincy, the soul is destroyed. If the Shinigami dies from any other cause, the soul is reincarnated in the human world. I believe even the cursed members of the Komamura family, including Sajin, were once human souls. So, as far as I know, all Shinigami are Souls, who once lived as humans.