That's why you approach them from behind.
We've all had a momentary lapse in judgement. This hunter was just concerned and wanted to check the welfare of this deer. Well, this buck wasn't having any of that.
According to the hunter's Facebook page, this deer was actually wounded the night before. The same guy that walked up to the deer actually had a decent shot on the deer, but somehow only managed to get one lung. After waiting 20 hours, this was the result.
All I can say is even if I see a down deer that looks dead, I still wouldn't walk in front of it. Remember, these are wild animals. To be safe, approach from behind. If it has its head up and you have a tag, don't take the chance.