Poison Spirit was in a bind. If Jiang Yi was really in the Demon-Buster Valley, he would definitely think of a way for the three ladies to be reunited with Jiang Yi. It was actually very easy to send them there; as long as he looked for Mo Lingqiu or Di Ming, it shouldn't be a problem. The problem was that Jiang Yi was dead; what was the point in sending them to the Demon-Buster Valley?
The Divine Tree's actions had made everyone think that Jiang Yi had perished. As Poison Spirit himself had gone to verify, he was very certain that Jiang Yi had indeed died. Poison Spirit had returned to Jiang Yi Prefecture to help Jiang Yi maintain this prefecture and help Jiang Yi protect his kin and loved ones.
If it wasn't for Jiang Yi, Poison Spirit would probably still be doing the bidding of Xuanyuan Lingyan and would be hunted down like a street rat once his identity was exposed; he would not be able to live openly for the rest of his life.