What is the significance of the quote "Whoever fights monsters should see to it that in the process he does not become a monster" in relation to the theme of morality and ethics in literature?

A man fights a monster must turn into a monster to kill it, if not he will not get sufficient strength and courage. After finishing his job he must return to his previous state but sometimes he may not able to return to his previous state. The possessed spirits which gave strength, courage and the mental state may not quit him. They may ask for more job to do.

There is an example to this in a Hindu legend of Narasimha, the fourth avatar of Hindu God Vishnu. Narasimha didn't return to normalcy after killing Iranya Kasibu. His anger and roaring created so much energy that planets were thrown out of orbit and crashed into one another. The earth trembled. Supernovas exploded in the universe and a inter-galactic cosmic fire erupted. Gods came together to bring him back. This legend is applicable to all humans. After killing a monster they must know how to bring themselves to the previous state or normal condition. They must forget everything as if nothing had happened.

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