What causes Garraty to almost lose it in Chapter 8, and how does McVries help him?

Garraty witnesses another Walker whose feet have been destroyed by the halftrack. Without feet, the boy can't keep up and his face is blown away. Garraty can't believe what he saw nor does he understand how the soldiers expected the boy to walk after running over his legs. McVries distracts him by asking Garraty questions about his girlfriend and then offering to tell him the story of his scar.