Mark of Cain
The Mark of Cain appeared on Cain when he took his brother Abel's life as punishment. In modern terms, people have a certain look about them that tells you they have bodies behind them. Blood is on their hands and it can never be hidden no matter how hard they try to hide it. It is wise to be able to know the look and keep your distance.
I met a weird guy at the bar who I just didn't like the cut of his jib. I couldn't tell what was up with him. After a few drinks we chatted a bit, and my suspicions were right. The guy has bodies behind him; he has The Mark of Cain.