Ever since Jiang Yi found out that he could control the Sword Emanations Race beings, he made up his mind to torture You Tianni to death!
That was his character. He would not provoke anyone who did not provoke him first. However, if others went out of their way to plot and scheme against him, he would ruthlessly kill them without any hesitation as soon as he got the chance.
That was what happened to Yi Wu, and that would happen to You Tianni as well!
Yi Wu was a talent from the Phantom Race. The fact that he was sent out to the Demon-Buster Army showed that the higher levels in the Phantom Race favored him. The Phantom Race perhaps wanted him to be nurtured. However, Jiang Yi did not hesitate to kill him. You Tianni's background was heaven-defying. What consequences would he have to bear if he killed him?
Jiang Yi did not know, and he did not want to know. He just wanted to kill him first before he thought about it!