User blog comment:Xilinoc/Ch. 614 - KILL THE KING/@comment-24574959-20150205181038/@comment-5398287-20150205184509

I, uh...I think we've known both of those things for a while now. The Soul King's purpose is keeping all the realms in existence by existing, and Yhwach wants to kill him because he wants to end his own/the Soul King's "humiliation", which (theoretically) is either being unable to live unless he absorbs souls or being a limbless entity who can't even move, respectively.