It was true that Ji Ning had made this trip with the intent of killing Empyrean God Greatdream, but in his heart he didn't have absolute faith in what Empyrean Gods Sin and Sealthroat had told him. Based on the reports he had seen prior to entering Undermoon Lake, those two were unlikely to be petty, vindictive liars, but Undermoon Lake had a way of changing people. In addition, Ning had grown extremely close to Empyrean God Zhenbu and other Seamless Gate members on Myriad Mountains Island, and so he wouldn't act against the Empyrean Gods of the Seamless Gate in a casual manner.
This was why he gave Greatdream a chance to sneak attack him!
He wanted to see for himself exactly what type of a person Greatdream was. Perhaps ordinary Empyrean Gods wouldn't dare to allow others to sneak attack them, but Ning had the [Eight-Nine Arcane Art] protecting him; he held no fear of Greatdream at all.
And in the end…Greatdream really had launched a sneak attack!