if the guard was handcuffed to the prisoner from the beginning of the trip, to prevent an escape, then who was driving the van in the first place?
did the driver park on the side of the road to take a leak or crap exiting the vehicle with the keys to the door and the cuffs. while he/she was relieving him/her self the prisoner killed the guard that was handcuffed to him thinking that the guard had the keys and he would be able to get free, and he is now angry and frustrated because he now realizes that the other guard had the keys.