“Who you are in the dark” is the trope name for how you act when nobody can see or know. Are you actually a good person, or are you only good when you’re being observed by people who’d judge you for being your true self. This is literally the same thing as “Santa knows if you’ve been good” and “God knows what’s in your heart.”
These kinds of people actually need an invisible, omnipresent judge to keep them from committing atrocities. They’re the people who, in a supernatural disaster movie, are the ones who turn to human sacrifice to try and appease whatever horrible thing is rampaging around outside or the alleged god who’s doing nothing to stop it.