Pang Shengwu had only occupied Hong Zhenxiang's body for a few years and had not yet completely devoured and erased Hong Zhenxiang's soul and will. However, he suppressed Hong Zhenxiang to the point where he had no autonomy whatsoever. Over time, complete dissipation was just a matter of time.
Now, facing Su Changkong, who is about to take his life, Pang Shengwu knew of the life-and-death friendship between Su Changkong and Hong Zhenxiang, so he deliberately released his suppression of Hong Zhenxiang, allowing Hong Zhenxiang's soul and will to take over the body in hopes of making Su Changkong hesitate.
If Su Changkong were to make a decisive move, both he and Hong Zhenxiang would die!
Pang Shengwu's tactic was indeed effective. Su Changkong had believed that Hong Zhenxiang was completely dead, leaving behind nothing but an occupied shell. But now, realizing that Hong Zhenxiang's soul had not perished, it meant there was still hope for salvation!