There are many factors and situations that can go wrong in all types of hunts. Rhino as well as elephant dart hunts are both very successful ways of raising money for not only the preservation of future elephant and rhino populations but also huntable populations of both elephant and rhinoceros. I should also note that dart hunts are still done on a permit basis. You are not allowed, even if you technically own the rhino, to dart that animal more than a small number of times in a given year. In most cases, when a dart hunt has taken place, a veterinarian will be on hand to perform routine check-ups on the animal which has been darted.
I got a bad taste in my mouth not about the circumstance in which this hunter and his PH were put in, but how you portrayed the events that unfolded. Saying the guy shot a charging rhino in the head sounds a lot less emotional then saying he shot it in the face as it struggled to get to it's feet. If you get a chance, watch an elephant hunt or a buffalo hunt. Often times, even when shot with a lethal shot, these animals will continue to charge. Until the animal has been completely dispatched, they still pose a strong threat to the hunter, PH, and anyone else in that hunting party.