User blog comment:Kaido King of the Beasts/Why Just Have Two?/@comment-24.235.56.170-20150425232636/@comment-25926617-20150426011209

That is not entirely true. We have seen him do it once, one does not make a pattern. He may just need them to stay still/ restrain them and since he couldn't do that to WB then he had to kill him.

This is my best guess is that he sucks all the DF power out of the person using the yami yami no mi's power so i don't see why he would have to kill them.