Motive remains unknown.
ISIS represents some pretty arrogant SOBs, don’t they?
The Islamic subjugation of all other ethnic cultures and groups everywhere is inevitable, in their eyes. They simply do not see the very real countervailing force that exists in the souls of the rest of humanity, when it comes to being forcibly converted to an incompatible view of the human condition.
Like all other forms of totalitarian authoritarian rule, the hierarchy is always uneasy, because they never know what smoldering anger lies buried ready to burst out at unexpected places and with totally unpredictable effects.