Lifeforms made of soul pieces were very hard to kill, and even if they were killed, the victims would be the owners of the soul pieces and not the lifeform.
If Wang Lin were to use celestial spiritual energy and held nothing back, he could kill this thing. However, more soul pieces would come and reform this lifeform again. In a certain way, these lifeforms were undying.
However, when the Cultivation Planet Crystal shatters, these soul pieces will also collapse.
No one knows whether the next lifeform they face will contain the soul pieces of their loved ones. At that moment, how would they act? If it was your own soul piece, then that would be easier because you have a mysterious connection with your soul piece.