As some sort of punishment or a form of sick "play" maybe? God knows what kind of depravities the King is into.
Thing is, the King would not bed under 15 because of the danger of a child death. So how could he possibly be allowed to kill a child and in such a way? Just doesn't make sense. As far as sexual pleasure. He looked really bored :/
Well remember the scene where Theo discovered that the King has been killing children occasionally (the one where he noticed that the birthmark on the child)? My guess is while the King did declare that he wouldn't be sleeping with children under the age of 15, that doesn't prevent him from doing something else to them. Maybe the kid died because he wanted to test how far the kid could take the pain or he did something to piss off the King? Who knows.
Well I'm lost. Why did he have that dog kill that kid? Idgi