Whoever fights monsters should see to it that in the process he doesn't become a monster

Whoever fights monsters should see to it that in the process he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you.

Whoever battles with monsters had better see that it does not turn him into a monster. And if you gaze long into an abyss, the abyss will gaze back into you.

Battle not with monsters lest ye become a monster and if you gaze into the abyss the abyss gazes into you.

Mais c'est renfantillage - this is childishness!' we heard de Grandin pant as we closed in and sought a chance to seize his skeleton-like antagonist. 'He who fights an imp of Satan as if he were human is a fool!'

("The Man In Crescent Terrace")

He who fights with monsters might take care lest he thereby become a monster. Is not life a hundred times too short for us to bore ourselves?