He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.
Source: In The Ultimate Success Quotations Library, 1997.
-- Friedrich Nietzsche, (Oct 15 1844-1900), German philosopher, poet; He was a noted critic of Christianity, conformism and nationalism who wrote The Birth of Tragedy, 1872.