The Venom first appeared on the final page of Amazing Spider-Man #299 (by David Micheline and Todd McFarlane) as a new kind of threat that brought back the alien symbiote from earlier stories and paired it with Eddie Brock to create a character who quickly became a fan-favorite villain. However, it didn't take long for the villain to transition to a new form and gain an anti-hero moniker that he's now carried across multiple mediums.
Spider-Man and Venom may have started as enemies, but they eventually made a tentative peace, and Venom ventured off on his own to become a "Lethal Protector." But how did he truly make the transition from villain to anti-hero, and earn that title?
After bonding with the symbiote and becoming Venom, Eddie Brock spent a couple of years as a dedicated foe to Spider-Man, hunting him down and typically being treated as one of the wall-crawler's most deadly threats. During the period where Peter Parker believed androids created by the Chameleon were his parents, Venom returned to bedevil Spider-Man once more. Kidnapping "Richard and Mary" in Amazing Spider-Man #375 (by David Michelinie and Mark Bagley), Venom was under the mistaken impression that he was protecting them from the dangers posed by Spider-Man. Peter's subsequent investigation leads him to meet Ann Weyling, the former wife of Eddie Brock.
Both end up finding their way to Eddie's hideout at the defunct theme park Thrill World, and Spider-Man once again faces off with Venom. But after the web-slinger proves himself a hero by protecting Ann in the ensuing chaos, Venom admits that killing Spider-Man would doom untold numbers of people in the future. Venom makes a deal with the wall-crawler, agreeing to drop his vendetta against him on the condition that he is allowed to escape and that Spider-Man won't chase after him. Venom ventures off across the country, and ends up making a home for himself in San Francisco where, in the Venom: Lethal Protector miniseries, he tries to figure out how to transition from a brain-eating monstrous villain to an anti-hero.
Venom became a controversial figure, and even though he was a dedicated defender of San Francisco and did his best to fight crime on the streets of the city, he also found himself not just targeted by villainous forces like the Life Foundation and the Jury, but also the police force. After briefly being captured, Venom was experimented on and used to create a number of spawns who would go on to become dangerous symbiote themselves. After escaping and learning of Roland Treece's plan to expand his power base at the cost of everyone living in the Underground City, Venom teamed up with a reluctant Spider-Man to stop Treece from killing numerous innocents.
Venom ultimately defeats and captures Treece, and even spares his life before escaping back to the Underground City and being given a new home. It's at this point that Venom truly became a Lethal Protector, dedicating himself to safeguarding others. Although he has occasionally backslid into villainy and the symbiote was used for dark purposes when it was separated from Brock, the character has largely been a far more heroic figure since then. This transformation is also notable for leading to the incarnation of Venom seen in the films Venom and Venom: Let There Be Carnage, with the symbiote spending much of the latter film openly wanting to become a "Lethal Protector" of the world.