Thing is, we do need to cull deer, as we killed off their natural predator, the wolf. If we don't, then deer will die of starvation instead, as our use of the land doesn't provide endless resources for a large deer population. However, natural predators don't on the whole kill off healthy stags in their prime, they go for the weaker members of the herd, which keeps the population of deer healthy.
So, yes, to kill this magnificent creature years too soon, and in the middle of the mating season, was a terrible thing to do. Removing the genes of the strongest from the pool is terrible, and as has been said above, all to stoke the ego of some idiot city boy on a hunting jolly. I don't believe that any person professionally involved in the deer hunting business would have committed such a stupid piece of mindless vandalism.
BTW, I'm not generally in favour of blood sports, before I get royally flamed, but the responsible hunting of deer, when the meat and hide will be put to use, and when done respectfully to control the population is an exception.