He who fights with evil must take care lest he thereby becomes evil in the process.
Do not let what you fight take you down a level. Stand above them, and take care to not get consumed by the heat of emotion; for it will drag you down.
He who fights with monsters might take care lest he thereby become a monster.
And if you gaze for long into an abyss, the abyss gazes also into you.
The fight you fight is both an outer and inner fight to keep the enemy at bay.
Friedrich Wilhelm Nietzsche