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.
If your goal is to terrify a nation in order to pressure it to change its foreign policy then it is quite rational. And there are quite a few examples of this approach actually working.
The point is that for a phycologicaly healthy and balanced human being these actions can not be called rational by any stretch of logic
Their actions could absolutely be called rational and logical if you were to accept their premise - which is what I'd argue is irrational.