It's irrational if you do it for kicks. It's rational if you expect a strategic advantage for your group/nation-state out of it. Whether it is moral to do is an entirely different discussion.

It's not irrational at all.

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.

Well obvioysly to someone irrational every of his action seem rational.

The point is that for a phycologicaly healthy and balanced human being these actions can not be called rational by any stretch of logic

I think you'll find that psychological health has little to do with what people can convince themselves of, or convince themselves to do - i.e. you can't explain away Daesh (or WWII nazis, etc.) with mental illness.

Their actions could absolutely be called rational and logical if you were to accept their premise - which is what I'd argue is irrational.

You don't even need a Godwin point for that. Nobody was calling Air Marshall Harris, well-known for his strategy of carpet-bombing civilians with incendiaries during WWII, "insane" or "irrational". Instead, they gave him a bunch of medals and made him a baronet.