A lot of scary looking people have what is known as "Gun Face." It's when you feel confident in saying that a person owns a gun and has brandished it or has a gun on their person purely from the appearance of their face. Their expression is unsettling- a mix of insanity,glee and contemplation, as if they're wondering if they've got enough old carpet left to roll their next victim up in.
Vin Diesel (if that IS his real name) has Gun Face.
I don't want to go to the post office. The guy behind the counter has Gun Face, and is probably going to snap any day now!