It is not OK anywere to have an animal to suffer for 40 hours before u finally kill it. If u are going to hunt and kill an animal u must make sure that it dies quickly. If u cannot do that u should not shoot.
It seems to me that he said "i want to kill Cesil, here is a lot of money, make it happen". I hardly belive that it was a coincident that it was this individual lion he killed. That kind of "hunting" has nothing to do with real hunt. It is not uncommon with rich people that pays to kill a trophy, often a trophy that is placed infront of their rifle.
Hunting is not about killing animals, its more about making sure u have a strong population that can live on for a long time. That mean u cannot just shoot whatever animal that shows up, it means that you leave the important individuals, like strong males and females w kids so that the population can grow stronger.
In case of lions it seems like a bad idea to kill an alpha male like this, it has a huge impact on the rest of the pack. If u absolutley want to kill a male lion, go after the ones without a pack.