I say "what" because the conventional liberal belief about John F. Kennedy's assassination tends to make the question "what" rather than "who," then wind itself back to "who" again, but not to Lee Harvey Oswald, whose guilt is inconvenient.
In order to avoid the generally accepted fact that Kennedy was killed by a left-winger, liberals have been required either to embrace unprovable and often very far-fetched speculations about the real killer(s), or to engage in something close to Orwell's "doublethink." In the latter, it can be admitted that Oswald pulled the trigger, but the real killer is held to be a set of abstractions--"hate," "extremism," "intolerance." These became incarnate in the city of Dallas as a "climate of hatred" which, by a mystical influence, became the real assassin, with Oswald himself only its puppet. And since there are a lot of right-wingers in Dallas, and right-wingers are full of hate, they must have been the principal generators of the evil climate. And therefore Kennedy was actually a victim of the right; the enemies of liberalism g0t the blame, and the leftward end of the political spectrum remained untainted by the intramural murder of one its less radical members regarded as a hero.
It's a neat study in the psychology of evasion. And it's still very much alive, as various "news" stories about the assassination showed on the occasion of its 50th anniversary last fall. Pundits do not ask whether leftist politics can overcome the stigma of the assassination, but whether the city of Dallas can. Right-wingers never can, of course.
That was going to be the introduction to a link to a detailed description in the March issue of The New Criterion of how the myth began to take shape immediately upon Kennedy's death. However, the piece, by James Piereson, is not online, so I'll have to content myself with a couple of quotations from it. Describing a news story on the front page of The New York Times flanked by an editorial by then-Washington-bureau-chief James Reston, Piereson notes that
Two narratives of the assassination were thus juxtaposed on the front page of The New York Times on the day after the event. One was based upon the facts, which pointed to Oswald as the assassin and to the Cold War as the general context in which the event should be understood. The other was a political narrative, entirely divorced from the facts, that pointed to "extremists on the Right" and a national culture of violence as the culprits in the assassination. According to Reston's interpretation, the assassination arose from domestic issues, with the civil rights crusade front and center.
The attentive reader would have noticed that there was a conflict between the two narratives such that both could not be true. He may have wondered which one would prevail in the days ahead as investigators sorted out the facts. If so, then he did not have to wait very long for an answer.
He goes on to quote various pundits and politicians, starting with the Chief Justice of the Supreme Court, blaming bigots in general and segregationists in particular for the crime.
The JFK assassination was thus an event in the Cold War, but it was interpreted by the liberal leadership of the nation as an event in the civil rights crusade. This interpretation sowed endless confusion as to the motives of the assassination and the meaning of the event. It made no logical sense to claim that Kennedy was a martyr in the cause of civil rights while acknowledging that the assassin was a Communist and a supporter of Fidel Castro. In deciding which of the two should go--the facts or the interpretation--many decided to eliminate the facts, or at least to ignore them.
That is certainly true of, for instance, this book review, and presumably of the book itself. It could almost be called unhinged in its focus on who did not kill Kennedy. It's as if a camper in Yellowstone or Yosemite had been killed by a mountain lion, and the rangers agreed not to mention the mountain lion, but to issue grave warnings about the danger of bears.
 Not that I am unwilling to consider the possibility that we weren't told the truth, or the whole truth. But if you're going to blame someone besides Oswald for the crime, you ought to have some definite evidence. And most of those who blame the right offer either nothing, contenting themselves with innuendo, or elaborate speculations that gave rise to the derogatory phrase "conspiracy theory," and that convince no one else.
 Not that I think most liberal journalists at the time were explicitly sympathetic to Oswald's Marxism, but they were indulgent toward the far left in a way that they never were toward the right. And the subsequent decade saw them moving further left, with more reason to embrace the myth.
 Not that it's absurd to link the assassination to a national culture of violence, because we are indeed a fairly violent nation, but most of our violence is not especially political (e.g. the long-standing high level of criminal violence in our big cities).