Not harm/hurt a fly
Definition 1 of 1
Pro Tip 1/3Gentle Character
Use this phrase to emphasize someone's peaceful nature without suggesting physical weakness or cowardice. 
My grandmother is so caring, she wouldn't hurt a fly.
Pro Tip 2/3Non-literal Meaning
The phrase is not about flies. It's an exaggerated way to say someone is non-violent. 
He's a pacifist and wouldn't harm a fly.
Pro Tip 3/3Emotional Emphasis
The phrase can also imply affection or protectiveness towards the person described. 
Don't yell at her; she wouldn't harm a fly.