Question: why black zetsu kill madara?
Answer: Madara didn't die from Black Zetsu. In the scene, Black Zetsu stabbed Madara in the chest and made him absorb a lot of chakra and converted Madara into Kaguya Otsutsuki. ... A person dies when their tailed beast chakra is extracted from them. So Madara died when the Ten Tails was extracted from him.