If you can't beat them, negotiate; if you can, just wipe them out! That was the mentality of those Monsters back then.
Just...
Where in the world is it so easy? You're allowed to fight when you want and refrain when you don't; such a situation might occur elsewhere, but at the very least it was absolutely impossible to take place in Zhou Tian's domain.
Therefore, had those Monsters come honestly to negotiate with Zhou Tian from the start, he might have actually let them off easily. After all, Zhou Tian didn't intend to annex them, and dealing with those Monsters wasn't something he was particularly enthusiastic about.
However, the former actions of those Monsters had infuriated Zhou Tian, and even though he was rational enough to know that he couldn't completely annihilate them, that didn't mean he wouldn't punish them.
