Silver Class 17
The team leader intentionally did this
Anyone can be tempted with such a stone so a hunter taking it without permission was acceptable. The hunter escaped but before he could far end from their eyes; the stone blasted also killing the hunter. The stone was not in stable state when the monster got killed definitely it has effects on the stone as well. For simple elaboration Qiao had seen through the flaw of this stone after observing the dying monster. His attention was to keep the safe distance yet the random hunter appeared out of nowhere and fulfilled his intentions unintentionally.