That is terrible that they killed the cougar , he didn't do anything wrong. When you are out camping you are in the wild and should know that there is danger.It is a shame that such creatures have to die in these situations, but they are killed as they have tasted human blood and will therefore be willing to attack a human with much less caution next time.
The problem is as HS says ... potentially dangerous animals such as bears and cougars can become habitualized and lose their natural fear of humans or see them as prey and for this reason they may be destroyed.
It is one of the reasons that stopping to take pictures of wild bears in the National Parcs is discouraged - you may be contributing to the bears death by doing it.