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Dethroning Moment: Harry Potter
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Law and Disorder: The treatment of Slytherin was a deal breaker for me even in the first book. Even reading it as a child I recognized that everyone in the school treated an entire quarter of the student body as pariahs. Wins are taken away from them or boo'd relentlessly, they're constantly reminded they're the 'evil' house, and this behavior goes all the way up to the headmaster (Dumbledore went out of his way to participate in this, making it clear he was taking away the House Cup earned through grades and good behavior because Harry and his buddies did things against school rules and after the contest was closed). When it's made obvious that everyone will hate them no matter what they do, and adults will not only not protect them but encourage and actively participate in the abuse, it's just self-fulfilling prophecy that they're going to act out and join organizations that shun the wizarding members who harassed them.
Ecojosh1: I hate the jinx that Hermione put on Marietta. Hermione secretly puts a jinx on a piece of paper that all DA members sign. When Marietta betrays the DA, the jinx doesn't give her amnesia or otherwise prevent her from betraying them. Instead, it disfigures her so the word "sneak" is permanently written on her face. I can understand the heroes wanting to identify the traitor, but disfiguring someone is horrible. If Hermione had warned people about the jinx, it might have deterred her from betraying them. But keeping it a secret takes away the deterrent aspect of the jinx and just makes it vindictive.
Remtar85: What makes this an even bigger DMOS is Rowling's explanation for it after admitting that Marietta's scars never really healed. Her reasoning? She "loathes a traitor". So basically her reasoning is that if a kid is a tattletale, they deserve to be scarred for life? What the hell?!
Animeking1108: Order of the Phoenix was so bad, that it made this troper Rage Quit the series for two years. I could write a list of reasons why it sucked the size of the padded book itself, but I'll just go with one dethroner. There's the reason as to why Marietta ratted out the DA. It wasn't because she had malicious intentions, like a grudge against Harry or a secret alliance with Umbridge. It was because she was under the threat of her mother losing her job with the Ministry of Magic. Then there was Cho's Informed Wrongness for defending her, getting kicked out of the DA and losing her relationship with Harry as a result. Yeah, how dare she defend her friend.
Gess: The SPEW sub-plot. If there could be a more insulting, tactless, off-handed way to tackle a sensitive and controversial topic, I'm not aware of it. Wizarding society has the practice of keeping elven slaves who are conditioned to obey their masters, find delight in serving them, abhor freedom, punish themselves for failing a task, and is not limited to corrupt and stuck-in-the-mud nobles, but is widespread and common. So it falls to the non-wizborn Hermione, a brilliant emissary of the more enlightened and advanced world, to get to the bottom of this issue and probably rectify it... Nah, let's just turn the whole deal into a fucking joke! Let's have Hermione come up with a stupid-sounding acronym (apparently, without realising it), act like an annoying, obsessive, self-righteous shit-stirrer, never bother to properly research the case and try to release the elves against their "will", thus looking even more deluded and in the wrong. And then, after the joke has served its purpose, let's forget all about it!
Fluffything: The whole subplot turned Hermione from the smart girl (who sometimes wanders into Insufferable Genius territory) to a Know-Nothing Know-It-AllFree the Frogs type character. It's such an utterly OOC moment for Hermione that there's no way it could be used for anything other than as a poorly-written joke. You'd think JK Rowling would handle the subject with more sensitivity, and would have Hermione handle the situation with care and thorough research on the subject. Nope, it's treated as a joke. The joke is that Hermione is trying to free slaves.
K9feline5: On the train back to Hogwarts, Harry sees Marietta and smirks because she still has the jinx. 'Til then, I hadn't been inclined to be sympathetic to her, but this one brief moment shocked and repulsed me out of the narrative. It effectively destroyed the possibility that there was a magic healer available to Marietta and her family who could remove the jinx from Order Of The Pheonix, and left in doubt any chance that any of it would fade. So this extremely minor, voiceless character who committed a single bad deed that failed will have that deed emblazoned across her face for all to see, forever. Other characters who did much worse are forgiven, but Marietta is not.
StormRequiem: Something that has always bothered me is the way the Ron/Lavender "relationship" is handled. Obviously, I wasn't expecting them to end up together at the end of the series or anything like that, but the way Ron treats Lavender disgusts me. Yes, she's annoying and clingy, but she's a sixteen-year-old girl in a new relationship. It's obvious she likes Ron more then he likes her and he basically snogs her repeatedly to prove something to everyone and make Hermione jealous. I recall Rowling saying something like she paired the two of them together because Hermione had a lot more relationship experience then Ron and she felt at that point he didn't "deserve Hermione"... So, to make him "worthy", she has him date a girl he obviously doesn't have genuine romantic feelings for, lead her on, then inevitably dump her for Hermione. And he doesn't even have the balls to actually do it to her face, he just lets her see him with Hermione and waits for her to break it off herself. How exactly does that make Ron worthy to date anybody?
Theenglishman: Remus Lupin trying to abandon his pregnant wife. Interestingly, Lupin's behaviour could be seen as an in-universe Dethroning Moment of Suck in Harry's eyes. Lupin had slowly become more deranged and paranoid since Voldemort's return, and combined with the guilt of both marrying Tonks and the chance that his son might be a werewolf, it culminated in Lupin's offer to join the Power Trio. Harry, realizing that Lupin had completely lost it, called him out and told him to grow a fucking pair, after which Lupin bolted from Grimmauld Place with his tail between his legs. True to the DMOS formula, it is the absolute nadir of his character and when we see Lupin next, he has indeed become more of a family man.
Samadhir: Near the end of Deathly Hallows, before the final battle of Hogwarts, Voldemort sends a message to the school that if they hand over Harry, they will all be spared destruction. This leads to Pansy Parkinson, a Slytherin, to immediately state that they should give in to his demand without a word of protest from any other Slytherin, nor any agreement from any non-Slytherin, prompting McGonagall and members of the other houses to immediately throw her and the entire Slytherin house out from the school. This moment has always angered me because it could have been an awesomeand moving moment for Slytherin if they had chosen to stand with their schoolmates and brethren, and finally earn some genuine respect from them. Instead, they're all shown to be cowardly backstabbers to the very end. It was a completely wasted opportunity for Rowling.
gallantmon son of darkness: I believe it was a dethroner because of them being thrown out of the school. I mean, they're a bunch of middle- and high-schoolers, of course they would choose to do that out of fear alone. How would they not be scared out of their minds when they're up against what is pretty much wizard-Hitler? What that poor excuse of a teacher should have done was either encourage them or give them a way to back out.
Rhysdux: Actually, that scene is even worse than Samadhir mentioned, because Pansy was the only person who said that they should turn Harry over to Voldemort. Immediately, the Hufflepuffs, Ravenclaws and Gryffindors all formed a wall between Harry and the Slytherin table and drew their wands...even though no one at the Slytherin table, including Pansy, was attacking Harry or was even threatening to do so. And given what spells witches and wizards can cast with their wands, the Slytherins were suddenly faced with all of their schoolmates drawing the magical equivalent of loaded guns and pointing the guns at them. Because one person cried out in panic. Then McGonagall treated the Slytherins as if they were a hive mind and were incapable of disagreeing with Pansy and threw them ALL out—odd behavior, given that at least one member of the Order (Andromeda Tonks) was in Slytherin. Finally, almost all fans end up blaming the Slytherins for being cowards and not fighting against Voldemort...when they never had a choice in the matter. What was that you were saying in the second book about our choices determining who we are, Rowling?
The Adept Rogue: The final battle between Harry and Voldemort is a huge DMOS to me. I mean, the hero finally faces the Big Bad in one final showdown, and all that happens is Harry delivers a "The Reason You Suck" Speech to Voldie, who refuses to listen. Both cast a spell, and Voldie dies. One of the most anti-climatic Final Battles I've ever read.