Fitzgerald is very deliberate in his intent. He wants the reader to become eyewitness to the murder, that is why Nick is not there to summerize the event. Fitzgerald does not want the reader to view Gatsby's death through anyone else's experience, he wants the reader to experience the death on their own. In this way Fitzgerald forces the reader to perhaps assess their own set of values perhaps imagining what they might have done if it were them in the novel.
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.
Fitzgerald knows that the death of Myrtle Wilson is coming up and he wants to make it a little suspensful that is why he puts all those refrences about death and he uses them in very specific ways he wants them to be noticed. The death of Myrtle was very grusome her breast was mamed so bad it looked like a flap of some sort and she was so badly injured in the face that it was horable. but gatsby wasnt the one even driving the yellow car it was Daisy Buchanan and the way she handled it was absolutly rediculous.