if looks could kill
In chapter XVI of Bram Stoker's Dracula in a diary entry of Dr. Seward, he notes about the undead Lucy Westenra that "If ever a face meant death--if looks could kill--we saw it at that moment."
An ellipsis (anapodoton) for an expression such as "If looks could kill, her look would have led to a murder of the person she was looking at."
A possible origin of the phrase comes from the legend of Medusa, who had the power to turn anyone who looked at her into stone.
- (idiomatic) Used to characterize a look of strong hostility