Spoilers for The Walking Dead comic book series ahead!
The creator of The Walking Dead comics almost made Shane the first big villain of the series, as the original survivor was considered for a much darker role. However, the comic book's creator, Robert Kirkman, revealed that had he chosen to kill Rick Grimes and not Shane, it likely would have led to the character becoming the first big bad of the series. The decision would have changed the series forever.
The Walking Dead is no stranger to introducing compelling, terrifying villains who make living in a zombie apocalypse somehow even more of a nightmare. From the Governor, who terrorized the series' characters and killed several major characters before dying himself, to Negan, who had one of the bloodiest introductions in comic book history before transitioning into a more heroic role, The Walking Dead is full of worthy villains. However, Robert Kirkman admitted he almost made Shane the first big villain in The Walking Dead, while he nearly killed Rick Grimes in the first arc instead.
In The Walking Dead Deluxe #6 by Robert Kirkman, Tony Moore, Dave McCaig, and Rus Wooton from Skybound Comics, Kirkman discussed one of his alternate plans to kill Rick Grimes, not Shane, as the first story arc concluded. The plan would have changed The Walking Dead in a significant way, as Kirkman wrote that it would have caused plenty of tense moments between Shane and Carl, who saw his dad get murdered. As a result, Kirkman said, "Shane would have been the first real big villain in the book."
As Kirkman wrote, the death of Rick Grimes, which could still happen in The Walking Dead live-action spin-off movie, instead of Shane, would have created an interesting dynamic of the original characters first big villain being someone who was in their group. So although Shane not becoming the first big villain worked out for the writer, as while he certainly transformed into a somewhat villainous role before his death, he was not the first big bad of the series. He simply couldn't bear Rick's return.
Whether Shane is really a villain is up for debate, but ultimately, in the six issues of The Walking Dead he appeared in before his death, his actions almost always had the group's best interests in mind. However, if Rick Grimes had died, as opposed to doing so later in the series, and not Shane, it could have led to him becoming more villainous as he tried to gain more control of the group after the main hero's death. However, he was never the big villain of The Walking Dead, although he came close to being it.