The term "mentally ill" is extremely subjective.
They're targeting the same people almost every nefarious organization targets loaners and those dumb enough (or naive enough), hence substitutable to manipulation. The sick part is that most people aren't radical - they just happen to stumble on radical philosophy such as the ISIS and are quickly sucked into it.
Is that mental illness?
Heaven is eternal bliss. Struggling with hatchets may not do. Next time they should use paper as a weapon and really struggle attempting to murder fully armed cops or SWAT teams for instance. Then, and only then heaven may be granted!