Fa Ying escaped right under Ye Fan's nose. Although Hua Wu was right in front of Ye Fan, and Ye Fan could have killed her with a stretch of his hand, he didn't do so. He knew that the best way to kill Fa Ying was to let Hua Wu and Fa Ying fight amongst themselves. Having split the demon's cultivation, their powers had diverged significantly. Hua Wu had the power to destroy the Earth but also possessed the dark power to kill Fa Ying.
"Ye Fan, I hope you understand, I'm not grateful for your mercy. You'll regret not killing me sooner or later!" Hua Wu showed no gratitude towards Ye Fan and even seemed ready to kill him at any moment.
