How and why did the dog die?

As he's finishing the prayer, King nudges him with increasing urgency, and then suddenly leaps out from under the bush, running straight for the guard post. David realizes that King wants him to run in the opposite direction and takes off, not even stopping when he hears a shot and King bark in pain ... and then silence. Once he's a safe distance away from the guard post David weeps, angry with God for allowing King to be killed, but then realizing that King sacrificed himself of his own free will.