The question of whether we should execute a violent sociopath like James Holmes or Ander Brevik, etc is a separate matter. I think it would be hard for any reasonable person who knew what Holmes did to ever feel safe around him, even if he were somehow declared "cured" of his illness. Because of the threat that he would represent to society I think that society could quite easily argue that executing him was a simple matter of self-defense. Then the issue of his mental state isn't even relevant, because it only matters that he is dangerous, and not whether he is acting out of insanity versus depravity.
However, what most people refuse to confront in a case such as this, is the clear danger of holding absurd beliefs. At the very least, the mere act of believing highly paranoid and nonsensical things often forces one to distort ones view of the rest of the world. When the world is constantly telling you one thing, but you desperately want to believe something else, such as that you live in a comic book, then you will have no choice but to distort the information from the world around you to conform to your agenda. Of course, it helps if you have a neurological disorder that generates your delusions "naturally", but delusions can come from the outside as well.
People who actually believe elaborately paranoid claims about the Illuminati and the New World Order, under the secret leadership of Muslim Marxist Kenyan-born Barrack Obama, coming to confiscate your gun and your Bible so they can throw you in a concentration camp are holding a belief just as insane as Holmes, even though they may not have the neurologic disorder that Holmes does. Of course, most right-wingers don't literally believe all of these things. They just don't like Obama and therefore they are potentially sympathetic to almost anything other people say that is critical of him, even if it's way out there.
However, the ones we have to be careful of are the ones who that that seriously, because they are the future Timothy McVeighs and James Holmes's of the world.