This kind of thinking is very dangerous and can be used to excuse all sorts of terrible behavior. We are all responsible for our actions. I can't prove that we have free will, but I feel it's important to believe it.
Obviously when you've gotten to this point, it's time to apply the law, not justify things away.
Acknowledging that we live in a material world and there are multiple causes for things is incredibly minor by comparison, and isn't an ideological basis for carrying out terror attacks. Causes aren't the same as reasons.
But I agree, it's important not to lose sight of the fact that he did make the choice to be here today.
[1]One of the many is HN-aligned Sam Harris: http://www.samharris.org/blog/item/life-without-free-will