""Blue Keeper did nothing wrong"" Well, maybe his methods are a bit crash, but there's a strong-ass monster going around killing people. And it also attacked his Junior (which obviously he cares for). Revenge & wanting to protect the other trainees justifies torturing XX.
The show's done this before. Dragon Keepers are clearly painted as the MAIN bad guys. But the monster boss is pretty evil too. As a fellow human, my allegiance is clear.
...unless we think that XX would be convinced to betray the Boss with sweettalking.