Wouldn't Hurt A Fly Meaning

(idiomatic) To be of very gentle nature.

Example: 1960, P. G. Wodehouse, Jeeves in the Offing, chapter XI
  Talking of being eaten by dogs, there's a dachshund at Brinkley who when you first meet him will give you the impression that he plans to convert you into a light snack between his regular meals. Pay no attention. It's all eyewash. [...] He wouldn't hurt a fly, but he has to put up a front because his name's Poppet. One can readily appreciate that when a dog hears himself addressed day in and day out as Poppet, he feels he must throw his weight about. His self-respect demands it.