Definition
This idiom is used to describe a situation where someone is glaring at another person with such intense anger that it appears their look could inflict harm.
She glared at him when she found out he had broken her new phone; if looks could kill, he would have been dead.
He was two hours late, and she just stared at him. If looks could kill, he'd be in pieces.
The look on my boss's face when I told him I messed up was unbelievable. If looks could kill, I wouldn't be here now.