While I'd be hard pressed to think of anybody who wouldn't agree with killing a rabid dog, I think we're moving away from your 'reluctance to kill' scenario and more into the 'are you physically capable of killing' area.

Having been attacked by a pack of dogs while unarmed, I can attest it's harder than it sounds to avoid being bitten, but since humans aren't designed to use biting as a primary attack, it's a somewhat different scenario.

As for the hesitancy, I would agree that most would be hesitant, but for an entirely different reason - how to attack it without being bitten, rather than a reluctance to kill.
Honestly, its a combination of the two. Most people are reluctant to kill. And the backup reason why people are ruined is, even if they are willing, its not easy to pull off safely. Yes humans suck at using biting as a primary attack. However, they are very good at grabbing and biting whatever they can pull towards them. They dont have to kill with their bite, just break the skin. They grab ahold of your arm, your hand, whatever dangly bit they can snag, and you are about to die. That makes killing them safely a difficult task. And since most people wont have a firearm close at hand, melee combat is the main source of fighting, and thats dangerous.