Why does dally look sick during the flashback when he had seen things like this before?

In that part of the flashback, Dally had just seen Johnny being killed by Bob, Ponyboy's assailant. Dally had already expressed concern for other deaths he'd witnessed before, so to show his horror and emotional impact at the death of his good friend would have seemed out of character and less compelling than showing the reader just how shocking and awful an impact Johnny's death had actually had on him