Lizzie isn't the only one though. It seems to be a recurring theme on Walking Dead that there's someone having trouble accepting that the walkers aren't "people" anymore and are dead. We saw it a little in Hershel and the Governor. Each of them may have responded differently but they all at one point found it difficult to understand life and death. Perhaps it's also bringing into question what separates them from people other than the change. They were human and alive once. People can be monsters like walkers too. What is it that separates them from us? They are trying to survive just like we are and are killing people like some people do. The answer is one: a heart beat, two: rationality, three and most importantly in this show: emotion and empathy. I think that they were really trying to stress that towards the end when they killed Lizzie. To remind the viewers that she isn't a psychopathic monster. That she was a confused person with a wrong understanding of the world. (They killed her because She wouldn't have been able to recuperate in that world because of the constant mental stress and traumatic experiences that she would continue to experience for the rest of her life)