The JFK Assassination Conspiracy Theory

One theory that is taken from another broad theory leads me to this conspiracy.  The theory of JFK’s secret service men turning on him is the one I’m talking about.  There is a theory that one of JFK’s Secret service men killed him.  The theory states that a secret service agent named Roy Kellerman shot him.  That he shot JFK with a pistol through the governor killing JFK at short range.  Roy Kellerman was sitting in front of JFK in the motorcade and could have easily killed him from that range with one bullet.  The problem and the point that makes me disagree with this theory is the fact that Roy Kellerman was sitting forward the whole time.  Also if Oswald was in a perfect spot to assassinate JFK, then why couldn’t it be him?  Even if Roy did kill JFK, someone would have definitely noticed the pistol in his hand.   This theory doesn’t make sense to me.