Keep in mind, symbols are objects in the story that are meant to represent a deeper meaning. Often, when looking for symbolism, it is helpful to look for prominent objects, characters, or even places that recur more than once in the story. It also helps to look for seemingly insignificant objects which the author seems to put undo attention on in a scene or scenes.
In this case, the guns in "Killings" should be looked at as symbolic. Consider first that there are three guns of importance. Strout's gun, which killed Frank, Fowler's gun, which killed Strout, and Willis' gun, which is not the impetus for any death in the story but is available to be one. Predominantly, guns seem to be the symbol for revenge. Strout killed Frank as revenge for sleeping with his wife. Fowler killed Strout for the death of his son Frank. If you go back and read the story again, you will also notice how much description is given to the guns. Fowler, before killing Strout, frequently grips his gun in anticipation, as if it is the object of relief for his pain, the object which provides him courage, and finally, the object which finalizes his act of revenge.
The suitcase Fowler forces Strout to pack (and Strout's clean home) is symbolic of Strout's humanity and normalness outside of being a killer.
Willis' wife's pain pills could be considered symbolic of the fact that she and Ruth both go to sleep and ignore what their husbands are out doing late at night, despite the fact that though it goes unspoken, they likely know.