Beware that, when chasing monsters, you yourself do not become a monster… for when you gaze long into the abyss. The abyss gazes also into you. - Friedrich W. Nietzsche
I first heard this line when I was watching Bourdain’s Parts Unknown episode in Los Angeles. A cop talks about how his job in East LA, and how in fighting crime - fighting monsters - he became one himself. A crooked, corrupted cop that has since changed his ways.
Yet I find this is a common narrative when people pursue money, status, or power. In the pursuit of these goals, they become monsters along the way.