I connect so much to this Mark Twain quote as I strongly believe that everyone has a dark side. There are a lot of quotes about the dark side of humans, and your dark side is not necessarily a bad thing..
Everyone is a moon, and has a dark side which he never shows to anybody.
Mark Twain
Also check Famous Quote 1 (Edgar Allan Poe) & Famous Quote 2 (Rumi).
Related: Directory of Mark Twain’s maxims, quotations, and various opinions (1).