While Venom himself started his comic book life as a villain, he’s become more of an anti-hero these days, and if that’s the plan for the character in the new movie, it makes sense that Carnage is the villain for the film.
The fact that the origin story of Carnage is closely linked to that of Venom makes him the obvious choice for a villain in the film.
While Eddie Brock, may be a criminal, he isn’t without morality, Carnage is a psychopath.