"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"

The quote is an alerting about the threats of taking part in a constant battle against wicked or unfavorable forces. The first sentence warns that when one is identified to eliminate darkness or negativity, there is a danger of becoming taken in by it and adopting negative qualities or behavior. The 2nd sentence is a metaphor that explains the fascination and destination that one might feel towards darkness or negativeness. It suggests that one need to watch out for this destination and not permit themselves to be attracted too deeply, as they might lose themselves while doing so.