"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"

This quote indicates that if somebody is continuously combating against unfavorable or harmful things, they need to make certain that they do not become like those things themselves. Additionally, if someone invests too much time focusing on the darkness or negativity of a circumstance, they can be impacted by it themselves.