In The Walking Dead, it's no secret Robert Kirkman was angling to kill Rick Grimes off pretty early. He thought the book might not last long, so he was fiddling with Shane murdering Rick when they went off to have the talk in Issue #6. In continuity, Shane was bitter Rick came back as he loved Lori and couldn't get over their affair, and as he attacked Rick, Carl emerged from nowhere and shot him in the neck.
Carl saved his dad, and as Shane lay dying at their feet Rick consoled his son, hoping he wouldn't get PTSD. But as Kirkman details in the deluxe edition out now, had he killed Rick here, it would've set up Carl versus Shane.
Kirkman admits that with sales ticking up and the idea of going past Issue #12 becoming a reality, he had room to possibly tell a longer story with Carl harboring revenge in his heart. His alternative plan was to have Carl stumbling into the woods, seeing Shane just as he was killing Rick. He'd be stricken with fear and silence himself for the foreseeable future, watching Shane pretend to be a good man living a lie.
"The focus of the book would have shifted to Carl to a certain extent as he was too scared to tell Lori what he saw, for fear Shane would kill Lori, too," Kirkman confirms in the Letters section. "And Shane wouldn't have known that Carl had seen him. So there would have been a lot of tense exchanges between them. Shane would have been the first real big villain in the book, and he would have been with the group, on the same side as them, the enemy sleeping the next tent over."
Now, for the comics, this would have been fascinating as it would have added drama and tension. Carl would definitely have been setting up a cat-and-mouse game, laying traps and ways to get Shane killed by walkers. This could have made him more ruthless and even resulted in a civil war with those who Shane saved and who felt no matter what, he was still a good leader. That personal approach would have been interesting as the monster in the camp, the wolf in sheep's clothing, has a different emotional ring to it than stumbling upon tyrants on the road like the Governor, Negan or the Whisperers.
This could have seen Carl triumph and slip into Rick's shoes thereafter, all the way up to the Commonwealth. It would have made him more battle-ready and who knows, maybe his mom would have lived, as well as his baby sister. The fact Carl lives until the end would also have made the Old Man Carl story so fitting as he'd have been the protagonist to have seen everyone unite and create a new, peaceful world. Plus, as a youngster, him finding romance with Lydia and building a relationship, thus connecting with a younger audience, especially as he'd have been torn with greater responsibilities as a leader.
It would have been more difficult on the TV show, though, as Andrew Lincoln was a darling since Episode 1. Also, had the AMC team followed this approach in Season 1, Chandler Riggs as Carl and Jon Bernthal as Shane would have had a lot of ground to cover. Rick killed Shane in the show so had he gone out this early, Shane would have been teaching Carl, making for some really uncomfortable moments. This might have eventually led to him killing Shane and Daryl possibly becoming the alpha in the pack, but what makes it so unlikely is Carl was too young to lead the show. If it went that bold route, though, some adults who were killed off may have found themselves staying on longer like Andrea and Lori. But ultimately, it's hard to see the series without Lincoln building the early foundation for the alliance before he departed.