Tim O'Brien's Going After Cacciato (1978) can be characterized by something O'Brien says in "How To Tell a True War Story," a chapter in his later novel The Things They Carried—"when you go to tell about it, there is always that surreal seemingness, which makes the story seem untrue, but which in fact represents the hard and exact truth as it seemed." Because the novel skips back and forth in time and place and, more important, moves in and out of Paul Berlin's less-than-reliable mind, we can only infer from two surreal episodes that Cacciato is (most likely) killed by Paul Berlin.
Certainty, like many things in war, is in short supply, especially in a novel in which the separation between reality and fantasy is often hard to discern. To answer your question, we must deal with "truth as it seemed" rather than truth as an empirical construct. For example, at the end of the novel, when the squad has cornered Cacciato in his hotel room in Paris, Oscar decides he cannot confront Cacciato and gives the rifle to Berlin. Shortly thereafter, Berlin hears a "monstrous sound" and
The sound spun him around. Suddenly he was on his knees. He couldn't stop shaking. He squeezed the rifle. He held on tight, but the shaking wouldn't stop. (p. 330)
The "monstrous sound" is the sound of the first round exiting the barrel, and Berlin can't stop shaking because the rifle itself is shaking him—when he "squeezed the rifle," he shakes because that is what happens when a soldier fires on automatic. The force rattles one's bones. The question is, does Berlin kill Cacciato in a Paris hotel room, or is it more likely, given the structure of the novel, that this episode is a reflection of something that has already occurred?
In the novel's final episode, just after the episode in the Paris hotel, we are now in the past and in the area where, earlier in the novel, the squad finds Cacciato soon after he goes AWOL. Something traumatic has happened because Berlin is described as having "the biles"—that is, he has lost control of his bowels after an incident, a not uncommon result of combat. Lieutenant Corson comforts Berlin by telling him, "It happens, kid. Sometimes it happens." And Oscar's comment, not as comforting, is, "Dumb ... Stupidest thing I ever seen." Berlin has done something other than just lose his bowel control. A few paragraphs later, we learn that Berlin has been searching the hill for Cacciato:
He remembered the fear coming, but he did not remember why. Then the shaking feeling. The enormous noise, shaken by his own weapon, the way he'd squeezed to keep it from jerking away from him. Simple folly, that was all. (p. 333)
The details in this scene parallel exactly the shooting scene in the Paris hotel—the monstrous noise, the shaking, the squeezing. This scene, however, which triggers Berlin's loss of bowel control and is also the "stupidest thing," depicts Berlin's accidental killing of Cacciato on a hillside in South Vietnam, not in a Paris hotel. Fratricide, the accidental killing of a soldier by a fellow soldier, especially in Vietnam, was not common but happened more than anyone would like to acknowledge.
Although this interpretation of these last episodes is certainly reasonable, it is, like the novel itself, an exercise in the seemingness that pervades everyone's perception of what happens in war.