I have reviewed footage where Rebecca was killed. In that scene we can see a man in uniform from behind walking toward Rebecca. His torso is visible except for the head. Moments after he apply poison on Rebecca's hand there is a head shot from behind.. Only two frames. I analyzed this head shot from behind and his torso and posture and compared it with all suspects, and come to the conclusion..

If the person who kills Rebecca is red John, then this person is Raymond Haffner :-( 

Not the red John I thought would be.