Shane takes the guns, hands them out like candy, and when he sees Rick and Hershel return with the zombies on leads, he decides it is time for a little show and tell. Shane takes aim at the female zombie and says if it was still alive, then shooting it should stop it. A few bullets to the torso only make it angry. Rick says it's enough, and Shane walks up to the zombie and shoots it in the head, so that her head is actually blown backwards. Awesome. He says that he's had enough: Enough of risking their lives for a little girl who is gone; enough of living next to a barn full of walkers. He says if they want to live and survive they need to fight for it. He heads over to the barn and starts unlocking it. Rick tries to hand off his zombie to Hershel, but the old man is shell shocked, either because clearly Shane has blown his mind. If Rick really wanted to stop Shane, he could have had T-Dog or Daryl do it, since they are standing right there. Hell, Daryl had his gun at the ready, I'm sure he'd have happily shot Shane if Rick asked him too. Seems like Rick isn't the clearheaded leader he has always purported to be.
The walkers start filing out of the barn, obviously attracted to the noise, and Andrea, Daryl, T-Dog and Shane start doing some target practice. Maggie can't watch, but nods at Glennn to go help. Shane even quickly turns and takes down the walker on a stick that Rick was guarding. Hershel is horrified, but we don't see which walker is really his wife or stepson, or any discernible difference in facial expression when they stagger out/get shot. Lori tries to shield Carl and Rick just stands there like an idiot. After the onslaught, Hershel is still on his knees like a broken man, Rick looks dumbfounded (not an uncommon look for him) and Shane looks disgusted that it had to come to this. We get another shot of everyone standing around, to mirror the one at the beginning when they all heard the rattle of the zombies in the barn.