I'm not sure which one, Bo Ya or Yang Kai, needs to learn a lesson here. It's the demon continent, so technically you don't need any excuses there. If you are stronger than someone else, you beat their ass as much as you want. On the other hand, there's the war against humans going on and Yang Kai is a human, so if he gets out of hand and murders demons left and right, it might be impossible for the demon side to spare his life. Even if they spared him, they might make that voodoo puppet out of him to keep him in check. So, Yang Kai might indeed be wise to cook up schemes to make sure it's not his fault. If he can say the other side was hampering his only reason to exist in the demon domain: fix the void tunnels, then it's natural he will kill such nuisances and they can only blame themselves.