You’re driving down the road, minding the speed limit and all other laws, and a pedestrian jumps out in front of your truck. He dies. Should you be charged with capital murder? After all, intent doesn’t matter, and you killed someone.
Your analogy bears no resemblance to the matter at hand. The victim in this case was laying prone on the ground with his hands behind his back and was shot in the back. In your analogy, you'd have to drive off the road and run over someone laying face down in the ditch and kill them, and yes you would be guilty. And intent is not always the reason why the guilty are guilty, it's called negligence.