He risked Dressrosa's citizens lives, he let Luffy, Law and Sabo escape and didn't help Doflamingo, the man who was holding the reins of the world. He had the power and the perfect scenario to stop everything.
He is not going to cause the end of the world just because the strawhats are good, so they are not going to use the ancient weapons for the sake of evil, and plot will favour them enough against Kaido and Big Mom