I have a friend who works in cheetah conservation in Namibia and is understandably really upset about this... but optimistic because she hopes that it'll teach a few people more about how poaching works in Africa.
A few points from her on this:
Bow hunting is basically the choice these days by poachers in her neck of the woods in Africa because a gunshot can be heard from much further away, but a bow is silent. So while the American guy's defense will be "I had a permit, I assumed everything was done correctly!", well that makes little sense. He likely knew what was up, but how they had to be quiet about it.
Killing lions like this in Africa is doubly bad because while there are always excess males due to pride structure, hunters always want the best specimens. As such, weaker male lions get to mate who otherwise wouldn't (as the best ones/ their offspring are killed), overall diluting the gene pool.
The reason it took 40 hours to kill Cecil is because after they shot him it was a bit late, so they went to bed to go track him further in the morning, leaving the animal in pain. Disgusting people.
Finally, I feel obliged to mention that there is a lot of good hunting in Africa one can do that really helps the locals and that they love. There are literally millions of antelope in Africa that are not endangered, and you can't carry the meat over borders, so bonus for the local villages where they can't afford meat and it gets donated! So you have to be fairly fucked up to decide you want an endangered species instead.