Cletus Kasady's backstory for Venom 2 is revealed thanks to Eddie Brock's notebook from Venom. Sony's decision to try and launch a cinematic universe of their own appears to have paid off. The Tom Hardy-led Venom just set the October opening weekend record, making a sequel almost inevitable. Through the use of its post-credits scene, Venom already established seeds for the sequel to grow as Woody Harrelson made a cameo as Cletus Kasaday, the psychotic killer who becomes Carnage in the comics.
It was originally just before production began on Venom that reports surfaced that Carnage would make an appearance. It was later reported that Harrelson had joined the film with speculation pointing to this being his true role. This began to see some fans expect to see Carnage fully realized at some point in the movie, despite him having no presence in the marketing. That is because director Ruben Fleischer is saving Carnage for the sequel.
The post-credits scene for Venom only teased Kasady's eventual escape from prison and his transformation into Carnage. As it turns out, Eddie's interview with Cletus lasted far beyond what audiences saw. ComicBook shared an image of Eddie's notebook that was on display at New York Comic Con, which details Kasady's backstory that will be used for Venom 2. Between a massacre in New York and killing his grandmother, the Disciplinarian Administrator at St. Estes Home for Boys, and a random girl who wouldn't go on a date with him, the notebook clearly establishes Kasady's bloodlust.
These details are instantly terrifying for fans and just goes to show how insane Kasady is, even before he becomes bonded with the Carnage symbiote. These multiple murders are villainous enough, but his twisted psyche is further highlighted by additional details. Eddie believes he has Oedipus Complex (the feeling of desire for the parent of the opposite sex) and is possibly the reason why he dug up his mother's grave. Before that, Cletus tortured and killed his mother's dog with a drill.
Kasady is a worthy villain for Venom and Eddie to hunt down based on his prior record and what he would surely do upon being freed. But, it would be the pairing of Kasady's personality and the power of Carnage that makes him a supervillain-level threat. Since this notebook page and these details aren't explicitly states in Venom, a sequel will likely retell parts of this backstory and maybe even show it through flashbacks. It could be difficult to do just that with a PG-13 rating, but Venom 2 isn't expected to be R-rated either. However it happens, Carnage will be the villain of Venom 2 and at least we now know some of his backstory that Fleischer and company worked out for him.