Nietzsche makes his remark about the abyss (in Beyond Good and Evil §146) just after cautioning the reader that someone who fights monsters risks becoming a monster himself.

That can happen to the man of ressentiment. He’s convinced that his various disabilities are caused by someone or something out to get him, and that if only the scourge were eliminated from the world all would be well.

If this is your attitude, Nietzsche is saying, you’re going to get really good at ferreting out the nasty parts of life, wherever they might be hiding, and you’ll uncover one hitherto unrecognized injustice after another: first racism, then structural racism, then elitism, then heteronormativism, ableism, lookism, microagression…. You may get to the point where you can see nothing but monsters.