I have no problem with responsible hunters either. Nor really with calling a spade a spade: breaking a mouse's neck causes almost instant death, which is why it is deemed "humane". I suppose if you can guarantee that your stunned mouse is unconscious and isn't going to regain consciousness before it's dead, I'd consider stunning humane, too, but you can't.
The people I know who break feeder's necks, do it by hand, and don't use hemostats.
As for who determines the humaneness of killing methods: vets, anti-cruelty laws, and government and laboratory guides to the humane keeping and killing of lab and food animals.
Possibly "cruel" has such negative connotations that "necessary cruelty" comes across as an oxymoron, but yes, I believe that sometimes causing pain is necessary or at least unavoidable.