I do not have enough info about the incident in Zimbabwe so I will not make judgment just yet.
Only you did pass judgment instantly and the initial reason why this entire shitstorm started, why this particular case even went public to begin with, is because everything you're defending about hunting wild life was not met. He did not have the necessary permissions to kill this lion. Last time I checked there was a chance that he had been mislead but the article I read about this fella (I think it was Huffpost so it wasn't necessarily entirely correct :P) stated that he had a record of hunting endangered species.
Edit: Oh, actually that record of his illegal hunting is confirmed
Actually the conservation status of animals he hunted were not "endangered" but rather vulnerable at the most plus it also depends on the individual animal since an individual animal can lose endangered status under certain circumstances like contracting certain diseases (rabies for example) or going on a killing spree of younger, more fertile members of its species or sub-species.
I will say different things were said about the incident but the real question is, did he know that the hunt was unlawful or did his guides dupe him? We will not know the answers because people keep focusing on the wrong things and keep over-generalizing lions as "endangered." I will say I would love to be that dentist's armed bodyguard for good pay, great dental (obviously), and a chance to take out animal rights zealots who would love to put my head on a stick for supporting any form of sport hunting even if it was charitable like most overseas safari hunts.
Also, the following image may be a little inaccurate (based upon allegation) about the dentist but it does good illustrating people's priority issues and the hypocrisy of Zimbabwe's regime. The other image is funny.