He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss, the abyss gazes also into you.

[Wer mit Ungeheuern kämpft, mag zusehn, dass er nicht dabei zum Ungeheuer wird. Und wenn du lange in einen Abgrund blickst, blickt der Abgrund auch in dich hinein.]

Friedrich Nietzsche (1844-1900) German philosopher and poet

Jenseits von Gut und Böse [Beyond Good and Evil], Aphorism 146 (1886) [tr. Hollingdale (1973, 1990)]
    (Source)

Alternate translations:

He who fights with monsters should be careful lest he thereby becomes a monster. And if thou gaze long into an abyss, the abyss will also gaze into thee.
[tr. Zimmern (1906)]

Whoever fights monsters should see to it that in the process he does not become a monster. And when you look long into an abyss, the abyss also looks into you.
[tr. Kaufmann (1966)]