Lee Harvey Oswald could have been hired by an organization to assassinate John F. Kennedy, such as the CIA or the mafia. Both groups have also been suspected of being behind JFK's assassination and using Oswald as a trained sniper. George de Mohrenschildt befriended Oswald in 1962 and is suspected of being his handler.
Specifically, de Mohrenschildt was thought to be working with the CIA in a plot to assassinate Kennedy. Oswald could have possibly been a hitman hired by the government in a larger scheme to kill JFK.