He(Madara) will thank Naruto again for taking BZ down.
( we already saw Naruto separating BZ by cutting Kaguya's arm. He won't leave him alive no matter what because he badmouthed Obito a lot )
I don't know what jutsu he would use to kill him it will surely be the most painful and he will use rinnen tensai to bring him back to life just to kill him again and then he dies of using it as well again. whew!