User blog comment:Kaido King of the Beasts/How Do You Kill Quincy Jesus? Part 2/@comment-5398287-20160419004037/@comment-5398287-20160419203812

Because (and I know this is gonna sound corny/punny but bear with me) it would completely separate his brain from his heart.

I've seen a lot of regenerators in fiction who can heal to varying degrees, but one thing just about all of them have in common is that unless they're literally barred from death itself (e.g. Deadpool having been cursed with complete immortality by Thanos, Lobo being barred from both Heaven and Hell for his crimes), they can't survive having their whole brain separated from their heart/the rest of their body because it governs the regeneration itself. Not saying Kubo has to work within those lines, of course, but so far I see no reason for him not to (unless all the Soul King parts are supposed to be unkillable).