I slightly tend to think that, since DG was targeting the two adults, and found Nathan in the way, he may have hit him hard to knock him either unconscious or dead (I think he didn't care which) in order to be able to handle the two adults whom he wanted to capture alive. Nathan was simply something of an obstacle that he wanted to do away with. He didn't mean (hopefully) to capture Nathan alive for torture later. So he had two options: either kill him quick so as to deal with the adults, or leave him in the house. If Nathan was alive when leaving the house, perhaps it's because he was hit unconscious.
KL probably fought hard either to protect Nathan or after seeing Nathan having been hit/killed. The amount of blood she lost in the house may mean that she was barely alive when leaving the house and may have been deceased upon arrival at the farm (which would be a little mercy).
Just maybe.
P.S. I hope that when DG showed irritated look at the verdict it meant that he objected to the prosecutor's theory regarding how/when Nathan died. If so, he should make a statement to tell the truth if he wants to appeal.