Besides the obvious like a Bald eagle or Condor, I dont think I could kill any bird of prey, even a hawk or vulture, maybe its just respect for another predator. I also wouldnt shoot a mountain lion or wolf unless it was attacking someone (then again who is it attacking) Same reasoning.
My friend wont shoot a big buck, but he will shoot a doe without a second thought. I guess everyone has their reasoning.
I think everyone would shoot a boar.