Apparently neither he nor the treants have any knowledge of the concept of ecology. If the phantom beasts are native to the forest then bad things atart happening if you deplete/exterminate too many of the species. The overly judgemental noble/lord/village chief trope in so many of these mangas is tiring. The cerberus is contained, he obviously has a business plan, why not get specifics? Not too mention his knowledge of animal behaviour (demonstrated by the cerberus interaction) would be invaluable for surviving or hunting other beasts. They could even string him along while milking all of that juicy knowledge from him... A cerberus guard dog would also probably be useful as well.