i don't think dumbledore told snape to kill him because when it comes to the crucial point where harr has to kill voldemort dumbledore knew he would have to support and help harry. i believe that snape killed dumbldore for selfish reasons. it is truly sad that dumbledore died because i was depending on him to help harry AND his friends true this i believe that not only harry would be going on the lourney of killing voldy but his friends also.