How does Virginia's goodness win over Simon's evilness?

I don't know if I would go so far as to say that Sir Simon is evil. I'd describe his ghostly activities more as "naughty;" however, in life, he did kill his wife for being too plain. That is quite evil. If a reader views this story as a battle of good versus evil, then Sir Simon is the story's evil bad guy, and the Otis family would be the heroic good guys. Virginia is the member that ultimately vanquishes evil from the Canterville Chase. The key difference from a typical good versus evil battle, though, is that Virginia's goodness doesn't defeat Sir Simon's evilness in any kind of pitched battle. She accidentally comes across the ghost late in the story, and she kindly asks him to stop pestering her family.

"I am so sorry for you," she said, "but my brothers are going back to Eton to-morrow, and then, if you behave yourself, no one will annoy you."

From this point forward, Virginia and Sir Simon have a cordial conversation with each other, and she learns what happened in his past and why he is haunting the house. As the conversation continues, Virginia learns that Sir Simon is actually quite tired of the haunting. He wants it all to be over.

"Please don't go, Miss Virginia," he cried; "I am so lonely and so unhappy, and I really don't know what to do. I want to go to sleep and I cannot."

He cannot enter his eternal rest unless he has Virginia's help. She must escort him to the Garden of Death. He will be granted access because he is with the pure and innocent Virginia. She agrees to do this, and Sir Simon is released from his haunting. Virginia's purity, innocence, and goodness is ultimately what allows Sir Simon to finally move on. Her goodness doesn't so much as overcome his evilness as it does guide him to eternal rest.