Black Zetsu's feelings were easy to understand.
If he and Otsutsuki Hagoromo didn't share the same mother, he would have violently cursed the Sage of Six Paths for refusing to stay dead after more than a thousand years!
To Black Zetsu, the appearance of the Sage of Six Paths was by no means good news; it was a catastrophic piece of news that made it feel as though the sky were falling!
Chapter 57