Still, its very important to recognize that no matter how good you are at reading people, and threat evaluation, you shouldn't let your guard down and rely too heavily on that gut instinct. There's no violence in the police dashcam video below, but it does illustrate the point perfectly.
Bad People don't always look like or act like bad people.
The man that the Officer was talking to in the video is Richard McNair, and Richard McNair is a cold blooded murderer. Just a couple of hours before this video was shot, McNair had escaped from a Federal Prison in Lousiana where he was serving two life terms plus 30 years for murdering two people and seriously wounding a third in a botched hold-up. Previous to this particular escape, McNair had also managed to escape from both a city jail and a state penitentiary.
McNair's story is pretty fascinating if you ever have a chance to read about him. Lots of near misses while he was on the run. After the Police Officer in the video above let him go free, McNair traveled all over the US and Canada for over a year before eventually being recaptured and returned to prison. He's very clever, and even more importantly has the ability to appear "normal", calm, collected, and non-threatening. Even the fact that he gave the Officer two different names while being questioned wasn't enough to set off warning signals.
He just didn't feel like a bad guy.
I wonder if the people he murdered thought the same thing?