The average person cant understand ISIS and other jihadists because they are insane.
ISIS belong to a large subset of Islam called Twelvers they wants to summon the 12th Imam (Muslim version of the antichrist) too kick off the end of the world. Why? Because according to them the coming of the 12th Imam triggers events that lead to the coming of the messiah, conversion of the nonbelievers, and destruction of the Jews.