The plot Yu He had carefully engineered would have indeed been perfect, should things have gone slightly a different way. Both Chu Mu and Duan Xinhe would have been killed by someone else because of him.
However, Yu He had underestimated Chu Mu's strength and Duan Xinhe's cunning. It had ultimately led to his own death. It was no wonder that when he was dying, there was a triumphant grin on his face, akin to a conspirator having attained his grand goal. Presumably, he knew that Duan Xinhe would definitely kill Chu Mu, and the same Duan Xinhe, after killing Chu Mu, would soon be struck down by the experts of the Elemental Gate. Eventually, the two of them were doomed.
Chu Mu knew that by now, Duan Xinhe would hardly have any options left. Yu He's layered plot would have saved his own life upon reflection. If he used half-demonization, he should be able to hold on until the experts from the Elemental Gate appeared.