While other characters like Bellatrix Lestrange, Severus Snape, and even Dolores Umbridge contribute to the conflict, Voldemort is the mastermind behind the entire war and the ultimate source of evil. His relentless pursuit of power, his desire to control death, and his hatred for Harry Potter drive the events of the book.
However, it's important to note that the book's complexity goes beyond a simple "good vs. evil" narrative. There are shades of gray, and characters like Snape have motivations that are not solely driven by malice.
Ultimately, Voldemort's actions and ideologies are the main driving force of the story, making him the most significant antagonist in Harry Potter and the Deathly Hallows.