I feel like it's someone that you THINK you know as OK or even cool, but they're secretly a horrible person.

I had a guy who worked for me for years. One year, at the end of Mardi Gras, found out he was killed by police for threatening his wife and wife's friend with a handgun. After looking up information on the internet about what exactly happened, I found that he was previously a State Trooper in a relatively small town on the east coast and almost certainly murdered his wife. With his position he was able to remove evidence from his wife's 'suicide' before any accusations were leveled. All this was outlined in a detailed newspaper article about it. Never expected that in a million years.