Is religious delusion a mental illness?

When religious fanatics leave church and go and load up with dynamite, then go to the nearest most populated area and start mouthing prayers and praises for some idiotic god while detonating their prize and killing or maiming many innocent people are they doing the work of god or of their own messed up mind! Remember that they are 100% religious fanatics not atheists or agnostics, but loving religious nuts who think gods and devils exist!