First of all, regarding the wholesomeness of the story, it's well executed. Sure, it felt short, as the art is gorgeous and the story is told beautifully, so having it ended so soon makes me feel down for a bit. But I think in length, it is good that the author decided to conclude here. (Maybe the author could try to extend for 1-2 more chapters extra, but the main story - the background of the myth of Osiris' death - wrapped up here is just right, getting any longer will probably ruin the good up till now)
Second, I ... don't think Horus will kill Set. I mean, his smile at the end where he said "Wait for me, Lord Uncle" did not look like one of revenge driven. I wonder if Horus also might have memories from the previous life. Even though when time was turned back, Horus was only 5, but he seemed smart enough. And in the previous life, he - like his father Osiris - liked to stick to Set quite happily. Horus will defeat Set to take the throne of course, since that's one of the thing Set hoped for; but killing Set and the two brothers would reunite in the underworld ... Well, call me a crazy shipper, but I have a suspicion that Horus would be beyond thrill to monopolize his "Unca Set" in this life.