The Abyss stares into the Abyss

[Prince of Cats]

"Whoever battles monsters should take care not to become a monster too, for if you stare long enough into the Abyss, the Abyss stares also into you."
—Friedrich Nietzsche, Beyond Good and Evil, chapter 4, no. 146

What a brilliant and terrifying idea: has anybody got any comments to make or any similar quotations to share or discuss?