@Bers_Ser: Nah, the concept is there, just this is what happens when times change. In the beginning, people thought monsters and the Demon King were evil and killed them for the sake of the world. The Trees of Life were also considered evil which is why they were cut down: for the sake of saving the world.
Flash-forward to when Kyou was summoned. People now realized their mistake, but resigned to just farming monsters in the classical subjugation way. Kyou introduces farming in ways that are common sense to us, but 100% alien to them. SS Monsters, like Bennu, serve as shield to protect the remaining trees and are defending against attacks since the Heroes were the cause. Naturally, to the natural order, the Heroes are considered "evil" despite their noble intentions. Zachuck, however, is stuck in the old ways and is unreasonable (possibly insane) and she wants to kill all monsters for the sake of humanity, source of sustenance be damned.
Sui is the daughter of the Demon Lord, so I SERIOUSLY doubt Bennu would have put Sui in danger. She seems to have a better idea of what's going on. MC is not a murderer either, and obviously monster killing is what caused all this, so bloodlust won't work here. MC realized that which is why he let Bennu go. He believes it can be reasoned with since he saw Bennu cry.
Your reasoning is not wrong under traditional concepts, but this is not a traditional manga. Not criticizing you, just hoping I answered some questions for you.