An easy way to tell a villainous (or sinister) character apart from the protagonists, other than giving him/her a Red Right Hand, seems to be making their eyes red. If they don't start out evil, there's a chance they may get Darker and Edgier over time or perform full-blown Face Heel Turn. It's not the same thing as the "red eye" phenomenon in photos, but if cameras could detect evil, these would probably be the first people they'd catch. In the progressive algorithm of "Oh Crap" with eyes, we have: Examples of Red Eyes, Take Warning include: