What does Kiowa say to O’Brien? What is the effect of Kiowa’s repeated attempts at conversation? Why doesn’t O’Brien respond to him? What is he doing?

In the book, O'Brien relates how he killed a soldier when he threw a grenade at the feet of the soldier. O'Brien describes the massive injuries the soldier suffered.
After the soldier was killed, O'Brien noticed that one of the soldier's eyes was a gaping star-shaped hole. With horror, he also noticed that the soldier no longer had a neck, and in the gaping wound, thick blood had pooled.
Although Kiowa tried to convince O'Brien that he had no choice but to throw the grenade, O'Brien was speechless. Kiowa also told O'Brien to stop staring at the soldier's corpse, and he suggested that O'Brien lie down for a minute.
When O'Brien still didn't respond, Kiowa tried to tell him that he knew how terrible he felt. However, Kiowa soon retracted his comment after getting no response from O'Brien. What Kiowa was trying to do was to shake O'Brien out of his shock.
However, Kiowa had little success in getting O'Brien to speak. As for O'Brien, he didn't respond to Kiowa because he was shocked, grieved, and disgusted. Although he likely knew that war often led to terrible choices, he still saw humanity in the Vietnamese soldier he killed.