Could be two-pronged:
-Bit of shock after trying to reconcile his belief that they are just sick with having watched Shane put half a dozen slugs into center mass (heart, lungs, etc) of a woman he knew and had it keep coming without flinching.
-Knowing deep down that it was the right thing. that no matter how much he wanted to hold onto his wife, stepson, and neighbors, they really were gone, and had to be dealt with. While they were in the barn and safely locked away, it was just a concept, a mental exercise. Once they were out, time to cut the crap and face reality of the situation.
Since 1 happened before 2, makes pretty good sense that he'd sit there in shock/grief while Shane and company took care of business.
Still don't think the Sophia payoff was worth the amount of time it was dragged out. It was a good end to that part of the story, but could have been handled better/faster. Hell, it's a bit meta, but even the CHARACTERS were getting bored by that plotline
While still sad, not as big an emotional payoff as it could have been, either. She had about as many lines Sunday as a zombie as she'd had in the entire series to date (probably doubled her screeen time), so there was almost zero attachment to the character, outside of her impact on the others.
Since we were beat over the head with the search, we knew who the little girl zombie was, but how many people can honestly say they even remembered what she looked like before she staggered out of the barn? If that same zombie had been the 4th one out of the barn instead of last, not sure how many viewers would have noticed until the characters checked the bodies and saw her face...