Falconer - I think an important word there is "radical." I would agree with some of what you are saying. It has also been my experience that Muslims that I know do not believe this, but I would argue that most Muslims living in North America are not radical Muslims. My personal opinion on this is that we have postmodernism to thank for that.
Also, out of curiosity, can you please explain your reasoning for claiming that the Bible (or God of the Bible) condones slavery?
To answer the original question, I would say that in my study of Islam, I do and don't understand what the bomber is saying. I think most Islamic clerics would say that part of the telos or goal of Islam is to create a worldwide Caliphate - that is that Sharia Law becomes the law of the land everywhere. This then will create a kind of worldwide utopia. So with this in mind, you could say that while the US does allow Muslims to freely practice their religion, the US does not allow their free practice of their religion to trump the US legal system (for the most part) or infringe on the rights of other Americans. Therefore, some radical Muslims would see the US government and most western governments as in opposition to Islam since it does not uphold Sharia Law, and certainly does not for everyone.
With regards to the Koran, my understanding is that most Muslims believe in the doctrine of abrogation. That is what comes later in the Koran trumps what came earlier on in the Koran. So the more peaceful early verses in the Koran are trumped by the more militant sounding commands to use any means necessary to promote Islam. If you are not a religious person or even a person for whom your religion shapes your worldview, it's difficult to understand how deeply beliefs like this can impact the way you live your life and what you hold to be important.
I still don't know if all that helps explain why the bomber would see himself as "defending Islam", but maybe it helps to understand a little better how his beliefs may have shaped his thinking and actions.