The Daily Beast carries a column which explains the rationale behind the indiscriminate use of terror by Sunni Muslim jihadis. It is their belief that by making the rest of the world hate and fear Sunnis, placing Sunnis beyond the pale, they will experience attack and rejection.
The world's Sunnis will be forced to turn to the terrorists for protection, as they did in Iraq, thereby delivering to the terrorists a huge fighting force. At some point what amounts to genocide - each side simply trying to kill all of the other side - could eventuate. Not an attractive prospect.