Old Devil Yu did not know about the Saintess of Hehuan Sect's private affairs, but her proposal touched him.
"Cooperating with Yu Linglong... it's indeed a possibility. Dealing with so many people single handedly is indeed challenging. But if Yu Linglong joins our side, everything would not be a problem," Old Devil Yu pondered: "Though, Human Sect has its own inheritance. If I cooperate with Yu Linglong, we will surely have to form a pact. Once the Heavenly Dao agreement is established, it would be hard to renounce it later. How would the Holy Sect develop then?"
Old Devil Yu does not wish for chaotic times. He was on the verge of being the most powerful in the world, of course he would want the world under his "leadership" to keep improving. This would facilitate his and Sky Devil Sect's continual rise in power.
However, as the leader of Sky Devil Sect, it's of course impossible for him to abide by the "Immortal Sects' Covenant" strictly.