Most species bare their teeth as a threat, a display of aggression, of leadership. It is a reminder that these clenched jaws can and will open your yielding throat. I want you to think of this (next time I smile.)
The Black Widow strikes. Excellent point about Loki at the end there, by the way. Loki is a master of a deception and the god of mischief, and she looked him in the eye and fooled him. THAT is amazing. Amazing!