The entire Coldpath dimension was a huge historical repository. Even though Hao Ren was not clear what kind of place it was at first, his two years of digging graves and his extensive experience in collecting bodies allowed him to determine the site's value. This led him to develop a relatively cautious and safe plan of action. Intuition told him that there were secrets hiding in these crumbling temples and that he wanted to keep them as they were if possible. Hence, he squashed the idea of using the Petrachelys and other weapons of mass destruction.
The demon king's army was strong enough to deal with this incident. And thanks to the strength of Coldpath itself, the battle between the demon hunters and the demon king's army would not damage anything. It would, however, be different if the Petrachelys and the armed UAVs were brought in.
His intuition was right.
