In the third installment of Harry Potter, Harry learns that he is being targeted by a murderer named Sirius Black who has escaped from the mysterious Azkaban Prison. Azkaban is guarded by Dementors, ghost-like beings that suck the joy out of you. Since Sirius Black is on the loose, the Dementors are too. They seem to represent evil itself, and Harry is terrified of them. They make him feel like he will never be happy again.
Harry is not that afraid of Sirius Black, but everyone else around him is. They try to protect him, but he makes light of the situation. He runs away from home after blowing up his aunt (like a balloon, not with explosives). He sneaks around the castle with the Marauder’s Map, given to him by Fred and George Weasley, which shows him where everyone is at any given time. He also sneaks into the nearby town of Hogsmead.
For Harry, evil is an abstract concept, even though he has already faced it directly a few times in his life. He doesn’t consider Sirius Black a real threat. His Defense Against the Dark Arts teacher Remus Lupin finds him with the map and takes it.
Lupin also tries to help Harry address his fear. When the class faced a boggart, a creature that turns into what you fear most, Lupin stopped Harry before he faced it, fearing it would turn into Voldemort. Harry said he assumed it would be a Dementor.
"Well, well... I'm impressed. ... That suggests that what you fear most of all is -- fear. Very wise, Harry."
The fact that Harry did not think he would see Sirius Black or Voldemort shows that he does not live his life ruled by fear of evil. He fears the Dementors because they represent what he has lost in his life. He has already faced too much death.
The threat of Sirius Black turns out to be not what it seems. Sirius Black is not evil, proving the theme that good and evil are sometimes in the eye of the beholder. Sirius Black was framed. His reputation came from the fact that he was supposed to have killed Peter Pettigrew, but Pettigrew faked his own death. He was actually an Animagus, and turned himself into a rat (Ron's Scabbers). Sirius Black was also an Animagus, and became a big black dog. They also find out that Remus Lupin is a werewolf.
The real evil in this movie is the same evil as in all of the Harry Potter movies: the dark wizard Voldemort. What changes is who is helping him. In this book, it is assumed that Sirius Black is helping him, but it turns out to be Peter Pettigrew. The movie tells us that the idea of good and evil is more complex than we thought. We assumed that Black was evil, but he was trying to protect Harry and stop Pettigrew, who was actually the threat. People also usually assume that werewolves are evil, but Remus Lupin was a teacher and a mentor. This did not stop him from losing his job when people found out he was a werewolf.
Note: The quote is from Chapter 8 of the book, but appears in the movie.