In my opinion, this is the most dangerous statement in this thread. Saying the actions of these people is irrational loses any ability to understand why they're doing what they're doing and how we can stop them. It turns them into a faceless enemy who are doing things because of hate, which is easy and makes my ego feel better, but doesn't really explain their actions or the actions of anyone. Nobody thinks they're the bad guy of their own story.
There is nothing more rational than terrorizing civilians to achieve a goal. It is the logical conclusion of rationality.
They did not terrorize citizens, they murdered them in the name of their non-existent god. Life will go on in Paris just as it did in New York. There is no goal other than blood lust and grim spectacle. All because of their nonsense and yes, irrational religion.