Death and time
Like all of Don DeLillo's books, Libra pays attention to the passage of time in the human experience. Specifically, that passage of time is shown through historicity, because the novel deals with real historical characters, John F. Kennedy and Lee Harvey Oswald. The question of time is a suggestion of death in DeLillo's writing, just like the reader probably knows about Kennedy's death at the hands of Oswald. The novel is about the problem of death.
Politics and community
The community is unified by horror when John F. Kennedy is assassinated. The novelist suggests that the assassination, the presidency, the tension with Cuba—they all play a part in the historical events. In other words, political turmoil is shown to be a multivalent, complicating process that leaves more questions than answers. Also, the event serves as a unique turning point in the political history of America, and the community knows it.
Plans versus execution
The novel features the question of one's intentions or plans, versus the execution of them (pun unintended). In this case, Kennedy's literal execution is symbolically tied to accidence, because the novelist suggests that perhaps the assassination was designed to fail, except that the sharp-shooter who killed Kennedy chose to kill him. This adds a question to the mix. Why would someone kill another, and what was the secret motivation for the change of plans? There is also the question of fate, which is undeniably present in DeLillo's treatment of the events.
