What is interesting about both of these short stories is the way in which both authors attempt to present the violence as realistically as possible, however, perhaps paradoxically, in O'Brien's short story, his desire to do that actually leads him to present it in a way that is unreal. Consider his description of how Mitchell Sanders is blown up by a booby trap:
Sharp grey eyes, lean and narrow-waisted, and when he died it was almost beautiful, the way the sunlight came around him and lifted him up and sucked him high into a tree full of moss and vines and white blossoms.
However, such a description only serves to reinforce the theme of the story, as O'Brien says that in a "true" war story, it is actually difficult to "separate what happened from what seemed to happen," and so our reality and perceptions are "skewed" and force us to believe that what happens is something different from what really happened.
By contrast, the violence in "Killings" is very matter of fact and direct. It is not glamourised or distorted by perceptions. The narrator seems to take a very objective, detached view on the violences. Consider how Frank's death is narrated:
Richard Strout shot Frank in front of the boys. They were sitting on the living room floor watching television, Frank sitting on the couch, and Marry Ann just returning from the kitchen with a tray of sandwiches. Strout came in the front door and shot Frank twice in teh chest and once in the face with a 9mm automatic.
Note the way that there is no emotion involved in this very matter-of-fact narration. It almost appears as if this is a statement or something in a court of law about what happened.
Thus I would want to compare the violence in terms of the narrator and how he presents it. In "How to Tell a True War Story," the violence and death in war is presented in a surreal way which serves to underline the theme of the story, whereas in "Killings," the brutal and simple realism is stressed in the matter-of-fact account we are given of violence.