At first, knowing only that he would be killed, I assumed that with such a complex plot managed by only three people, somewhere along the way there would be a miscommunication or misinterpretation leading to Oswald killing Kennedy. However, as I read further, I found out that Mackey himself, who was in on the whole plan from the beginning, gave the order to Ferrie to recruit Lee to kill Kennedy and told Raymo directly to hit Kennedy in the head. In light of this, what could have gone wrong with Win's original plan? Mackey would appear to have no motive to kill Kennedy; like Parmenter and Win, his motive for contributing to the plan at all was to force further US involvement in Cuba. To achieve this, it would probably be better for their shooter to miss, because if someone who could be traced to Cuba were to actually kill the president, it might cause an over response from the government. Unless I am a very poor reader and have missed something, DeLillo reveals no other reason for Mackey to kill Kennedy.
The only reason DeLillo states that might cause Kennedy to be intentionally shot is that, as Win says, plans tend to take on a life of their own and move toward death. However, the "life" of the plot is not revealed to us. All we know is that Mackey avoided contact with Win and Parmenter and at some point independently chose to kill Kennedy. Any reason is left to the imagination. In a way, I think that is the point. DeLillo's picture of the Kennedy assassination is an interpretation of extremely vague evidence partly to point out that almost anything could fill in the details from a fairly simple explanation to the most complex conspiracy, but we will never know exactly what happened. His conjecture as to what may have happened must exclude certain details, because the most complete picture we will ever get, even if we were to have all the material that DeLillo's character Nicholas Branch has, will not include every detail. Motives especially would be hard to piece together, so it is only natural that these are the things Don leaves most unclear.
Another point I think he's making with this deliberate lack of explanation is that plots really do pick up a life of their own. With a plot as complex as the one he described (and much more complex ones have been proposed), any little error could easily cause an unwanted result, and any member contributing to the plot can direct it in a new direction, much like the reason for Win's plot in the first place: the failed Bay of Pigs, where one person's decision caused the whole plan to fail. The only difference between this and Win's plot is that instead of the result being a failure, it was a more severe success. Much like Oswald, Mackey leads a complicated life, and much like the official story (and DeLillo's) says that Lee killed the president despite all the reasons he seemingly would not, Mackey seems to randomly lead the plot to where it ends up killing Kennedy. The reason for this must be unclear, because it is unreasonable to expect every part of such a complex plot to reveal itself, even if that part is a person's decision, just like Lee is too complicated to fully comprehend in anyone's interpretation.
