Red threads
Zatiel's plan was simple and highly effective. First, he took advantage of the explosion covering the group to possess a Divine Legend.
While he could have done this at any time, he chose to do it when Gilligan ambushed the group less than a kilometer away from Saint City, decreasing the amount of time he would be supplanting the Divine Legend.
He was the flash that pierced into the explosion of azure fire and possessed Orin, using his power to forcefully push the Divine Legend's cultivation to the Supernova Domain Stage.
From the beginning, his goal was to reach Limbo, so he needed to commit a sin grave enough that they would immediately arrest him, but not so great that they would kill him on the spot. Using a Demonic Path of Power was the best choice available.