I think I might have mentioned that the image of guns on a cover is not seen a whole lot. Even in the mystery novel genre the artist uses less obvious an more powerful imagery to capture the readers attention.
People kill people.
Artist will use a lot of methods to get this message across. The antagonist will be looming, or there might be several of them crowding in, or a figure just behind a curtain or doorway. But sometimes the artist wants to push the point of just who the antagonist is. They will make that person the only individual in the composition. And maybe the artist will want to make another point, something very obvious, maybe they will add a gun to the shot.