I was thinking watson sees him jump but not die ( because he says at the grave "I need you to stop been dead" )
At the start i thought that the 4th assassins sees him jump and then gets in the way so that he landed on him, The blood on
the ground is his but the body is taken away before watson becomes conscious but not sure about that because its a big jump
Then i was thinking that it was related to the iou. Maybe the girl reporter is the real villian and the Iou is related to the "I repel you" that holmes says to her ( think that was it )
So maybe holmes repelled down, the assassin came over touched him and then the 4th one shot him in the head, the blood on the pavement is splattered like a gun shot  ( and again taken away before watson arrived )