The main purpose of Randall is to serve as a cipher for Rick trying to keep his humanity when it's really a bad idea. He saves Randall without thinking things through. As a result he now has a new problem. They realize dropping Randall off in the middle of nowhere is almost crueler than killing him, but when it comes time Rick still can't do what's necessary and put a bullet in his head. And no doubt they're going to suffer because of that decision. The entire point of the storyline is to show that there's no place for humanity anymore, in the same way the Sophia storyline was there to prove there's really no hope left for them. It's about as black and dark a TV show as I've ever seen. Shane is the right man for the right time, but Rick doesn't want to become that man. And it's all because Carl is watching him - when at the end of the day he's teaching Carl exactly the wrong things too.
While it's an interesting view of the show, I don't really think that's what it's about. It's a morality play at it's heart and it's offering up both sides of the argument. You clearly think Shane is right. But I really don't think that's going to be the ultimate lesson of the show. Saving Randal in the first place was a bad call by Rick simply because it was a reckless endagerment of his life (along with Glenn and Herschel's). But I think it was made very clear that the kid had zero desire to go back to the group of people who left him to die. If he had, he would have let Rick and Shane drop him off, not said a damn word about knowing Maggie, and found his way back to his people and then led them to the farm.
If not putting a bullet in Randal's head comes back to bite them, it's going to be for one reason. Because after saving his life they proceeded to put him through physical and mental torture for several days/weeks. Which is a result of Rick losing his grip on his compassion and humanity and moving more towards Shane's way of thinking. If the Randal story has been about anything, I'd say it's about the group losing their footing and starting to fracture in the wake of Sophia's death. Rick saves this kid and instead of helping him mend and working to acclimate him to their group (something that probably would have been pretty easy for a lot of reasons), they tie him up, throw him in trunks, chain him in barns, beat the living crap out of him, threaten to kill him, put a gun to his head, etc. And as they are getting more and more reckless as a group, Carl is also getting reckless, wandering off into the woods and provoking zombies.
So yeah, basically I don't think that the ultimate moral of this story is that when everything goes to shit you have to abandon everything just to survive. It would be an interesting story, of course, but (and I know you've read the comics, Jim, but I'll use spoiler tags)