A phrase that you could say on a regular basis that would seem normal, but subtly hint at that sinister evil lurking within you.
As an example, imagine a situation where someone mentions an act of wrongdoing and you, very coldly, comment "I guess, everyone has a dark side". It's not a comment that is going to get you regarded as an evil person but still give an eerie vibe to the listeners.
I expect some creative phrases from my fellow sociopaths.