« If you stare into the abyss, the abyss stares back at you. »
Is Nietzsche caution to the reader that one who fight monsters risk becoming a monster too. If you spend your whole being fighting for a cause, never stopping, you stand the risk of, when looking around, seeing nothing but monsters to be slain. The monster is a figurative one, it can be societal stains like racism or gender equality, worker rights or even governmental overreach. By continuously trying to right perceived wrongs, the individual becomes a monster itself.