My theory is twofold: Fitzgerald knows a reader's biggest climax is in his or her mind and has the convenience of not having an omniscient narrator. Nick, of course, was not there when Gatsby was murdered; therefore, he is going on the information that he knows (as he does throughout the novel). Ironically, Nick's description of the "laden mattress" that was moving "irregularly down the pool" lends itself to Nick's vivid imagination, I suppose. Still, I think Fitzgerald takes advantage of Nick's limited point of view here because Fitzgerald knows just like a more modern writer, Stephen King, has written about. King always leaves the worst of the worst up to a reader's imagination because he knows that a writer could never match it. Not that Fitzgerald and King have much else in common (ha!), but in this instance, I think Fitzgerald wants the reader to imagine Gatsby's death. In this regard, I can also appreciate how Gatsby's and Wilson's deaths are handled in the movie version with the white curtains of Gatsby's mansion flowing and billowing in the wind, . . . so you are not quite sure what you are seeing. Fitzgerald: a master of his craft.