Evil All Along

That depends on the stories, because the heroes thought the villains is one of the good guys turns out to have always been evil.

For example, the hero meets a very nice person who offers to help the hero For whatever reason. If the movie is constantly trying to deceive the audience like this, it can be harder for the audience to get to know the villain.

This is when the character was always bad and faking good, not a fall to the darkside.

Disney Animated Canon

 * In Atlantis: The Lost Empire, Rourke and the other mercenaries (everyone on the expedition but Milo) were Evil All Along. Fortunately a Downer Ending is averted when everyone but Rourke and Helga do a Heel–Face Turn.
 * In Tarzan, it turns out Clayton was planning all along to capture and sell Tarzan's gorilla family.
 * Coco has the famed singer Ernesto de la Cruz, who is revealed to be a murderous fraud who lacks real song writing talent.
 * Prince Hans from Frozen, he only pretended to love Anna so he can take over the kingdom.
 * Assistant Mayor Bellwether from Zootopia ends up being the sociopathic mastermind behind the plot to drug predators into turning savage in order to turn the prey species against the predator population of the city.

Animated

 * In the Kim Possible movie So The Drama, the boy for whom Kim falls turns out to be working for Drakken. And a robot.
 * Stinky Pete from Toy Story 2 seems nice enough at first but is so obsessed with staying in mint condition for exhibition in a museum that he'll stop at nothing to keep Woody from having second thoughts, even if it means manipulation and threats, and especially harbors resentment against space toys such as Buzz Lightyear.
 * From the Pilot Movie of Buzz Lightyear of Star Command, there's Warp Darkmatter, Buzz's supposedly-Dead Partner who is in fact The Dragon to Zurg and had been on his payroll since the academy.
 * Lotso from Toy Story 3 has a warm-looking personality, but is revealed to run the daycare center out of the other toys' fear, and later on in the garbage dump leaves Andy's toys for dead after his life gets spared.