While I agree that there are just some animals that should not be kept as pets, I think saying that we should kill them the minute they do anything "wrong" (our perception, but quite natural from the animal's point of view) because exotic animals should not be kept as pets is just as wrong as keeping them. The reality is that they are here in our lives. People will always be fascinated with "keeping" them, and if we take away the resources to house them once they are no longer good pets, we just make it easier for those same people to acquire new young ones that are still cute and "innocent". People need to advocate and educate on this topic.
Years ago, dogs were thought to go feral if they ever bit a human being once, with the "taste of blood" driving them. We have learned since then. We figured out that dogs bite for a variety of reasons - fear, aggression, pain, hunger... the same is true for these big cats. I feel very bad for the young woman's family. I feel bad that she made the mistake of going into the enclosure alone without first securing the lion where he could not get to her.
I also hope that the police are better trained... and this speaks to handling dogs also. I can't tell you how many times I have read a story about how a panicked cop shot a barking dog because s/he was afraid of a bite. I have worked with and lived with dogs since I was 5. Only once did I experience a dog bite out of aggression (working at the pound), and even then the dog was able to release without too much damage. I know maulings happen, I have seen the reports. But when you look into the situation around the mauling, you find the little things that hinted at its coming...
We don't know nearly enough about these huge exotic animals (just a fraction of what we know about domestic dogs, and look how we can screw them up) to let our guard down for a moment. I keep reptiles. I have a few snakes that are or will be quite large when full grown. I also know that I am always learning about them. I know my limits, and I know how to handle my animals. I also know that I do not know nearly everything about them. Once in a while, my completely mellow lizard will turn around and bite me for seemingly no reason... then I find out later he was scared or sick or smelled a predator/the opposite sex on me. If that young woman had just been cleaning someone else's enclosure, the lion might have felt threatened from the other animals's scent. He may have been hurt or feeling sick. He may have felt aroused if there was another lion nearby in season... We won't ever know for sure, but I think it's important to remember when dealing with "wild" animals. Even the ones kept in the best of conditions can have an off moment. I think it sucks that our first reaction is still to kill something, then ask questions later. I had hoped that as a species, we had evolved further...