Even the most powerful creature must keep certain less-powerful inferior creatures alive just to reinforce their own sense of being the most powerful. The superior-inferior relationship only exists because of the two necessary components having been met, which are that which are superior and that which are inferior. If the inferior aspect were to disappear, so too would the superior aspect. Inferior creatures serve as a symbol and a constant reminder for the superior creature to know, be aware, and understand its own greatness and to acknowledge the power difference. If the superior creature were to eliminate all of the inferior creatures, there would no longer be anything for it to compare itself with or to anymore. Hence, the superior creature may get (or is likely or eventually) bored and thus the initial stages of its own destruction begins. Since there is no actual physical external threat present, the only threat that exists is its own mental state.
"And then, as I got her message, there came a light from her eyes - powerful beams of light." - Nikola Tesla