The zombies in the barn did have to be killed, in my opinion. That barn was not that secure, and it was foolish to keep them because they are not going to get "cured" and they are dangerous. Shane was right about that, though I think Rick was trying to approach it more diplomatically with Hershel.

What I love about the show is not the zombie aspect, I think it's just a masterfully done look at human nature and how people will behave in an extreme situation.