I'm not sure why they didn't try to save Saddler. Maybe it was a sudden thing. Or they didn't know much about it because they weren't very close to him. David didn't kill Saddler, though I do wonder what he did with the body. It was kind of weird, and I think KA might have just done it to send him further past the moral event horizon. Of course this was after he thought he killed Tobias, so there wasn't much point to it if they weren't going to go anywhere with it.
David would have made a nice villain, and I would have liked to see him narrate some books. But unless it's a Chronicles book, only the Anis get to narrate. So no David, James, Toby, Erek, Ardiss, or whoever.