When Harry Potter and the Goblet of Fire was released, there was a lot of debate about whether or not it was appropriate for children. The entire Harry Potter series is about a battle of Good vs. Evil, but there's certainly a lot of evil going on in Book 4. The complete lack of concern Voldemort shows when he kills Cedric (whom he calls "the spare") is unsettling for readers of any age group. J.K. Rowling has been very clear that she wants parents to decide for their own children whether Harry Potter is right for them. But she has also stated that she has a moral purpose in showing Voldemort's violence for what it is:
I have an enormous respect or human life. I do not think that you would read either of the deaths in that book [of Frank Bryce and Cedric Diggory] and think, yeah, well, he's gone, off we go. Not at all. I think it's very clear where my sympathies lie. And here we are dealing with someone, I'm dealing with a villain who does hold human life incredibly cheap. That's how it happens: one squeeze of the trigger. Gone. Forever. That's evil. It's a terrible, terrible thing but you're right, I know where I draw the line. Other people will draw the line in a different place and they will disagree with me. (source)
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In Harry Potter and the Goblet of Fire, with the introduction of the three Unforgivable Curses – the Cruciatus Curse, the Imperius Curse, and Avada Kedavra – we learn some interesting nuances of wizarding law. It's not only illegal to cause someone torturous pain (Cruciatus) and death (Avada Kedavra); it's also unforgivable to try and dominate someone else's will (Imperius). It's obvious that the Imperius Curse is wrong in the hands of power-hungry maniacs like Voldemort, who uses it to try and make Harry dance to his tune during their wizard duel in Little Hangleton. But there's also a moral problem with Barty Crouch, Sr.'s efforts to control his Death Eater son through the Imperius Curse. Even if Barty Crouch, Sr. thinks he's doing his kid a favor by keeping him under house arrest rather than leaving him in Azkaban Prison, can you imagine the horrible emotional damage it would do to both father and son to have a father exert that kind of daily power over his kid, for years on end? And isn't it part of Barty Crouch, Jr.'s problem in the first place that he feels under his father's thumb all the time? While Barty Crouch, Sr. may have had good intentions, his use of this illegal curse leads to disastrous results. Wanting that kind of power over another person, for whatever reason, is always going to lead you into dark places.
Harry Potter and Voldemort are similar in many ways: they're both orphans, both Parselmouths (wizards who can speak to snakes), and both were raised in the Muggle world. What separates Harry from Voldemort is that Harry (among other things) has friends whom he treats as equals. In Harry Potter and the Goblet of Fire we see how Voldemort treats Wormtail, his servant who's scared and desperate enough to cut his own arm off to bring Voldemort back. Voldemort lets him bleed on the ground for a while before finally giving Wormtail a new, magical arm. By contrast, Harry has devoted friends who help him because they love him: Dobby brings him gillyweed for the second task, Hermione helps him learn a Summoning Charm for the first, and Ron lets himself get cursed over and over again as practice for the third. Harry's friends freely volunteer their support in the challenges Harry faces. Voldemort has to rely on power and intimidation to keep his supporters in line.
Fear generally makes people stupid in the Harry Potter novels. A great example of this is Cornelius Fudge, the Minister of Magic. In Goblet of Fire, he's so committed to his denial of Voldemort's rise that he refuses point blank to believe Harry's eyewitness testimony. What's more, he rejects all of Dumbledore's excellent advice about not letting the Dementors maintain control of Azkaban Prison (this right after watching a Dementor eat Barty Crouch, Jr.'s soul before they can get an official statement from the guy). The Death Eaters thrive on fear because it causes social division and confusion among their victims. It also makes people like Cornelius Fudge easier for the bad guys to control.
The most obvious show of principles in Harry Potter and the Goblet of Fire is Hermione's foundation of the Society for the Protection of Elvish Welfare. Hermione has Very Strong Views about what should happen to house-elves, and she immediately expects everyone – wizards and house-elves alike – to flock to her message. She has to find out the hard way that social change doesn't work that way. Even if she does have some right on her side, she can't just wave her wand and make the whole world change to suit her moral standards. Another principled character (in a horrible way) is Barty Crouch, Jr., who has absolute faith in his master, Voldemort. If there's one lesson we can learn from Goblet of Fire, it's that believing strongly in your own principles doesn't necessarily make you morally righteous or correct. Principles can be misguided, even if you believe in them wholeheartedly.
In an interview after the release of Goblet of Fire, J.K. Rowling stated,
[B]igotry is probably the thing I detest most. All forms of intolerance, the whole idea of "that which is different from me is necessarily evil." I really like to explore the idea that difference is equal and good. But there's another idea that I like to explore, too. Oppressed groups are not, generally speaking, people who stand firmly together – no, sadly, they kind of subdivide among themselves and fight like hell. (source)
Even though wizards already have enough trouble trying to keep themselves secret from Muggles, the Death Eaters have to make everything harder by creating a hierarchy within the wizarding world between "purebloods," "Mudbloods," and "blood Traitors." And then non-Death Eaters have to go and give in to hatred of other groups too: Hagrid is a target because he's half-giant. Ron is a target because he's poor. Harry is a target because he is powerful and different. As Hagrid puts it, "everythin' seems ter happen ter you [Harry], doesn' it?" (18.54). His fame makes Harry an object of hatred for some wizards, including Draco Malfoy.
We can't forget, in the middle of all of these global themes of good vs. evil, that Goblet of Fire focuses on a bunch of fourteen year olds. In addition to the coming war with Voldemort, these characters must also face a sudden rush of confusing romantic feelings. Not only that, but Ron has to learn a Very Important Lesson about not giving in to jealousy. While these conflicting feelings of love and envy are rushing around, Fred and George decide to keep people laughing with their Ton-Tongue Toffees and Canary Creams. Although Goblet of Fire tackles plenty of adult themes, there's enough teen angst and wacky hijinks to keep Goblet of Fire from getting too heavy.
Harry's darkest times in Goblet of Fire all come when he's alone. When Ron isn't speaking to Harry and the whole school has turned against him before the first Triwizard task, it seems as though nothing can go right. Later, when Harry must face Voldemort alone at the end of the novel, he's absolutely sure that he's going to be killed. There's a clear message in the Harry Potter series that being alone makes everything worse. Whenever Harry is isolated – or when he isolates himself by trying to withhold things like his curse scar pain – all of his burdens seem a hundred times more dire.