WARNING: The following feature contains SPOILERS For The Incredibles 2.
Like all good superhero movies, Incredibles 2 features an unexpected villain twist. In the third act, the sequel's villain, The Screen Slaver, is revealed to be a puppet of Evelyn Deavour; Elastigirl's benefactor is the true power behind the mysterious tech-based villain, who can turn any screen into an outlet for "his" mind-control technology.
While this twist becomes obvious in retrospect given the character's name (a pun on the words "Evil Endeavor") Evelyn's motivations are far more compelling and mature than those of Syndrome - the villain of the original Incredibles. Ultimately, Syndrome was a boy who never grew up, seeking revenge against all superheroes because he wouldn't be allowed to become one. By contrast, Evelyn hates superheroes for a more philosophical reason.
Audiences learn the history of Evelyn, her brother, Winston, and their family early on in Incredibles 2. Mr. Deavour was an eccentric billionaire who loved superheroes and acted as their advocate, even having a direct hotline to two superheroes - much like Commissioner Gordon's red phone to Batman. Sadly, the laws that outlawed superheroes came into effect on the same night that a group of burglars robbed the Deavour mansion. Despite having a secure panic room, Mr. Deavour's faith in superheroes was such that he kept trying to use his special hotlines to call his friends for help; the burglars ended his life when they discovered him on the phone.
Winston and Evelyn (respectively voiced by Bob Odenkirk and Catherine Keener) inherited their father's company and turned it into one of the biggest electronics and media firms in the world. Evelyn was a technical genius, finding new ways to modify old technologies and develop new ones. By contrast, Winston was a people person who used his social skills to find ways to promote what Evelyn invented. Together, the two developed their father's business into something far bigger than he'd ever dreamed of.
The action of the movie begins when Winston, inspired by the reemergence of Mr. Incredible, Elastigirl and Frozone in their battle with The Underminer, reaches out to the heroes regarding a public effort to change the laws banning superheroes. This plan involves outfitting Elastgirl with a host of new gadgets, designed by Evelyn, including a new motorcycle that is as flexible as Elastigirl and a costume with a built-in camera that will record everything she does. With Winston putting his salesman skills to work on selling the idea of superheroes as a good thing and video evidence that contradicts the mainstream media depiction of superheroes as glory-hogs more concerned with showboating than saving lives, it is hoped they can eventually change the laws and let superheroes come out of the shadows.
The film's third act reveals Evelyn's malevolent intentions and how she took an entirely different lesson from her father's death than Winston. Rather than sharing Winston's belief that superheroes are just good people who just want to help others, Evelyn sees superheroes as a danger to society because they encourage complacency; she thinks that superheroes are symptomatic of their society, which encourages people to be lazy and expect others and technology to solve all their problems for them. Ultimately, Evelyn believes, perhaps not unjustly, that had her father would have survived had he just hidden himself away or fought the burglars himself rather than waiting for a superhero to save him.
This leads to Evelyn's creation of The Screen Slaver - a villain who turns people into the literal mindless zombies that Evelyn believes humanity is slowly devolving into. This reveals the twisted irony of the story of Incredibles 2 and Evelyn's plan. In order to thwart her brother's plans and save humanity from the threat of superheroes, Evelyn transforms herself into everything that she hates - a super-villain and a manipulator of the masses.