I've always wondered the same thing! I rewatched this episode yesterday on a whim, and I've come to the conclusion that the "monster" the old man is talking about that is going to tear up the ground and swallow everything is himself, since at the beginning he seems to come out of the ground before he's seen. Also since he brings up genetic evil, he's reborn as that kid in the end who I'm presuming turned into said "monster". Buuuut like I said, I don't really know for sure