To be honest, if he didn't have the special trait of chasing souls and taking lives, it would be very difficult for Zhu Wuwan to kill a Fusion Heavenly Pillar even if she advanced to become a Human Immortal.
Although the Soul Locking Gu could also deal with the Fusion Heavenly Pillar, the commotion it caused was too great. As long as the other party was prepared, it would be difficult for Zhu Wuwan to successfully insert the Soul Locking Gu into the other party's soul.
Moreover, it would take a certain amount of time to set up the Soul Locking Gu. During this period of time, if the Integration Heavenly Pillars discovered it, the other party could completely escape.
Just like how Zhu Wuwan had used the Soul Locking Gu formation to deal with the two Heavenly Pillars of Nirvana Tower, if the two of them had not greedily kidnapped Tang Yelan, Zhu Wuwan could have used Tang Yelan to locate the two of them and intercepted them many times before attacking them.
