He deserves to live because no matter what else, he is a human being with the rights and privilieges that implies. As soon as there starts to be exceptions to human rights (oh, he's a BAD man. He doesn't deserve to be protected by the rights the rest of us has.) then there's an opening towards adding more and more categories to the list of people who don't deserve the treatment human rights demand.
More importantly... it means he wins. If we start becoming a more Americanized society as a result of this, then he wins. He manages to make us worse people and our country a worse place to live. The only way to combat people like this is to become even better. To reach towards even greater heights of ethical, political and judicial standards. And yes, that applies to him and his ilk as well.
We already protect society from people like him by throwing his ass in jail or into mental institutions where he can get humane treatment as is his right. We protect society by reforming those who can be reformed, instead of turning them even worse through a shitty justice system. We protect our society by creating a society where people have less and less reason to become criminals or terrorists.
I am a Norwegian and I am DAMN proud of the fact that he will get treatment, isolation from society and be dealt with in a humane and ethically responsible manner.
Unlike the way certain other nations deal with both their own citizens and foreigners for far lesser offenses.