Even if you are reading this scene from Harry Potter and the Philosopher's Stone for the first time with zero context of the rest of the story, you don’t need to second-guess that Voldemort is the villain of the series. Rowling makes that unquestionably obvious by following this mythological template:

The archetypal definition of evil is that which corrupts the most innocent thing.

The unicorn, a symbol of purity and chastity in European folklore and medieval Christian symbolism, is Voldemort’s sacrifice — an irreversible moral corruption — for immortality. Hence, “a terrible price.”

Evil is the stepmother in Snow White poisoning the pure-hearted princess. It’s Cruella de Ville in 101 Dalmatians making coats out of puppies. It’s Pennywise in It, preying only on children and feeding on their fears. It’s the greed and manipulation of the Capitol in The Hunger Games, making a spectacle out of fights to the death. It’s the force of consumerism in Wall-E that leaves Earth overrun by waste and poisoning its citizens with insidious apathy.

So, the next time you want to write a character who radiates undeniable villainy, let them defile what is most innocent and pure and make them the antagonist of virtue itself — define evil through the very values they choose to destroy.