For John, BLUF: Mr Oswald did it. Nothing to see here; just move along.
It was a tragedy. We have no way of knowing how things would have turned out by 1968 if President John F Kennedy had lived. We hope better than they did. In fact, we have no way of knowing how things would have turned out by 1976 if Richard Nixon had won in 1960.
Mr Lee Harvey Oswald did it. It appears it was his second go at assassination. The first being an attempt on the life of retired Army Major General Edwin Walker.
What we do know is that people have focused on all sorts of alternative answers. Some see a conspiracy. Some even blame Dallas itself, as this post, "Remarkable Take on JFK’s Assassination", by Mr David Bernstein, addresses. The place for this post is, appropriately, The Volokh Conspiracy.
May the President Rest in Peace.
Regards — Cliff