Hershel was just doing what most of us probably would. If such an apocalypse were to occur would you have the ability to kill your loved ones, people you've known your whole life and loved? Once you've gone trying to save them you think you can do it for everyone else. Hershel and Co. were just doing what they thought was right, yes, but they had little knowledge of the apocalypse. From what we can infer, Hershel has not seen the zombies kill the way Rick and the Atlanta Group have. Can you blame him? He was doing something risky, but I don't think any of us would find it easy to kill loved ones.
Shane was being a douche to his group. The group was perfectly safe on the farm, but Shane might have gone and messed everything up. The shane could have easily fortified the barn and gotten to stay had it continued to help Hershel. If the group gets kicked off the barn, it's Shane's fault. Shane is becoming a control freak, Rick put him in his place at the end of the episode.