not hurt a flea
To harm nothing or no one. Said of one who is particularly gentle, shy, diffident, or timid by nature. My brother is a very sweet, warm-hearted man who wouldn't hurt a flea. How can you suspect him of committing this crime?
not hurt a flea
Fig. not to harm anything or anyone, even a tiny insect. (Also with other forms of negation.) Ted would not even hurt a flea. He could not have struck Bill. Ted would never hurt a flea, and he would not hit anyone as you claim.