At the beginning of "The Sniper," the narrator describes the civil war being waged between the Republicans and Free Staters. He describes constant gunfire and says that everywhere "the heavy guns roared." The guns of course do not literally roar, but the metaphor implies that the guns themselves are angry. The word "roared" also lends to the scene a sense of animalistic aggression. This metaphor is important because it helps to immediately establish the ominous tone and violent mood of the story.
Later in the story, the Republican sniper shoots the enemy sniper and then watches that same man fall from a great height, hitting the ground "with a dull thud." Looking down at the dead body, which is now no more than a "shattered mass," the sniper is disgusted and is immediately "bitten by remorse." This is another example of a metaphor, as the sniper here is not literally bitten. This metaphor is used to suggest the sudden physical sensation of pain which the sniper feels. This in turn emphasizes the degree of remorse felt by the sniper.
The sniper later becomes angry and throws his revolver against the roof. The gun fires as it hits the floor, and a bullet flies past the sniper's head. The sniper becomes momentarily afraid before the "cloud of fear scatter[s] from his mind." There is of course no actual cloud, and so this is another example of a metaphor. The cloud imagery works well here, as it is easy for the reader to imagine a cloud disappearing quickly and thus easy for the reader to understand how quickly the sniper's fear leaves him.