Clevinger and Yossarian are arguing over the enemy's intent when they "disagree" over what's happening. Yossarian is specifically plagued with the fact that the enemy is "trying to kill [him]", while Clevinger responds that they're simply trying to kill everyone. To Yossarian, this makes no difference, because he is the one currently in their sights.
The main point of this argument is an overall commentary on the state of war. The only way to truly survive the effects of war are to dehumanize the enemy, which is what Clevinger does. Clevinger just says "they're trying to kill everyone" so that it's almost an impersonal matter, and by killing them in response, he is doing a humanitarian act. Yossarian, however, feels personally targeted by the attacks and can't separate himself from the fact that there are individual humans (like himself) out there that are specifically seeking to kill him.
In chapter 2, Yossarian tells Clevinger, "They're trying to kill me." He is referring to the enemy...
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