Herschel was hurting. He was struggling within because there was still a part of him that believed that the walkers were people. But I think that his witnessing what took place in the past two episodes, being locked in with the infected, having to personally take some out to save some of the others, made him realize that he was only fooling himself. Made him see the truth.

I think that the humanitarian part of him was torn out when he came to the realization that the walkers are no longer people when they reach that state of being, but only monsters.

Herschel is a great character..... a very very deep character.