A plot twist is an interesting trope to say the least in fictional writing. On one hand, this trope has to power to make a normally okay story great and completely memorable. Moments such as the evil intentions of The Wizard in the book Wicked or the death of L in The Death Note manga series are such plot twists that have proven the trope's potential massive appeal on audiences. When used right, a plot twist can forever cement an audience for your brand of storytelling. However, there is then the negative aspects of the trope that make some people label it as a book destroying cliché. These instances are twists just for the sake of having one and events that deconstruct the story's entire setup. Such twists are the uncalled for break-up of Bella and Edward in New Moon (which I consider one of the strangest books I have ever read) as well as revealing Hollyleaf to not be one of the powerful three in the Warriors book Sunrise. When used wrong, a plot twist universally can destroy an entire storyline. The negative side of plot twists is what will be covered here in this part of a very long clichés editorial, particularly the character betrayal twist.
Character betrayal plot twists have to be one of the most overused tropes in not only the fantasy genre, but in all of fictional writing. Universally, character betrayals almost always ruin an entire character in the storyline, turning an interesting, complex figure into just another one of the cliché mustache twisting bad guys. In just a couple of pages, your complex wizard character who struggled with the idea that the world may be cruel and fought with demons in his head to prevent a complete mental breakdown can turn into a one note world destroying plot device. Your brave knight, who fought so long and hard to protect the kingdom he was raised in turns into a plot piece the second he decides to go rogue by joining up with the fire breathing dragon. Even the reverse side of this trope, a villain betraying the bad guys to help out the main cast, can be harmful if done wrong. Say you have a villainous werewolf whose evil intentions were the relatable theme of getting rid of persecution towards the supernatural. Although their method of doing so is in the wrong, enslaving entire human villages after ruthlessly invading them, the basic idea of their intentions intrigue the audience. Thus, the audience has a complex villain to be entertained with and the heroes have a true foe to fight. That all is taken away when the werewolf switches to the side of the heroes, all because they have a crush on the main character and are reduced to just a love interest. These examples are proof towards the destructive power complete betrayals have on the complexity of heroic or villainous characters.
There is also another factor to this trope's danger; having a twist just for the sake of shocking an audience. People would know this side of the character betrayal's danger on a storyline if they have ever watched some type of an M. Night Shyamalan movie. In his more recent films, plot twists have become a shameful part of his reputation as a director and script writer. Although he does not add these twists to purposely enrage an audience, these examples are complete detractions from the memorable films he helped create in the beginning of his career, such as The Sixth Sense. Basically, character betrayals can have the same type of notorious effect if used for the sake of shock. To me, Annie Leonheart in the first season of the anime adaptation of Attack on Titan is a prime example of character betrayals being used only for shock value. In my opinion, there was no need to add that twist into the storyline. There was already enough conflict and other plot twists coming forth because of Eren's reveal as a Titan shifter. Annie was a well established character that was serving a great role as a rebellious member of the corrupt Military Brigade. Revealing her to just be another man-eating Titan was unneeded at the time. It wiped her out of the cast after she froze herself in ice and seemingly ruined her character, just for the sake of shocking the audience. I am sorry to anyone who enjoyed this character betrayal, but to me it did not ring right. If not for the awesome fight scenes this twist brought forth, I would completely detest this decision by the creative team. Another character should have filled this spot, perhaps an already bad person whose character would only be expanded upon by revealing the existence of other Titan shifters. It then would have became a great example of the character betrayal trope, not one of the many cliché ones out there. Just food for thought. (You can roast me in the comments later if you do not agree on my stance. It is just my silly personal opinion. I know a lot of people actually like this twist, so I would not be surprised if I got called out for what I said here. However, I will not argue about how you feel on my stance. People are free to have their own personal opinions, even if it is not one I agree with.)
Do not use this trope at all unless you are really sure your character can stay complex after the shocking change of pace. It can be extremely damaging to a storyline if used wrongly, especially if you wish for your story to be loved by your intended audience. Beforehand, make sure you planned this event to occur from the beginning of the story and have a well established reason for using this trope. Otherwise, avoid it like the plague. Challenging the audience's expectations can be a great tool in writing. It brings them to places they would never have thought possible in the storyline, making every future moment more tense and more unknown. If you only do this though for shock value, be prepared for the audience to give negative criticism towards the entire storyline, even if most of it was well received.