Terror and Revenge
One of the problems with random acts of terror is that proportionate response doesn't seem to exist. How do you punish a suicide bomber? How do you discourage the like-minded? You can't always, but the ancient answer was to slaughter all the perpetrators relations to the ninth degree - in practice, those of his village and the surrounding villages.
That seems barbaric, by modern standards, so instead we hold whole countries responsible, and slaughter random samples of same. The improvement isn't obvious.