Another way is through a series of tragic events. If Naruto experiences multiple losses, such as the death of his closest friends or mentors, he could become disillusioned with the world. Writers can show how this grief and anger transform him into a corrupted version of himself, as he starts to question the values he once held dear.
Some writers justify it by creating a situation where Sasuke has to make a great sacrifice. For example, he might give his life to protect Naruto or the Hidden Leaf Village from an overwhelming threat. His selfless act in these scenarios is seen as a way to atone for his past mistakes and show his growth as a character.
Well, often they use the idea of Naruto's strong will. They say that his will is so strong that it breaks the normal limits of what is possible in the Naruto universe. So, his will allows him to access these impossible powers. Also, in some fanfictions, Naruto goes through some extreme and unique training or experiences that no one else has had, and this is what gives him these extraordinary abilities. For example, he might travel to a different dimension where the laws of physics and chakra are different, and when he comes back, he has these new powers.
Fanfiction writers could use power imbalances. If Naruto has suddenly gained a lot of power and doesn't fully control it, he could accidentally hurt Sasuke. It shows the consequences of power and the need for control in their relationship.
They could also use the idea of being influenced by dark powers. Maybe Naruto comes into contact with a powerful and malevolent force that slowly corrupts him, making him do things that are out of character for the original Naruto. This gives the writer a way to explore a different side of the character while still having a somewhat believable reason for his actions.
They might justify it through misunderstandings. Maybe Arthur is tricked into a situation that looks like cheating but he's really trying to protect Merlin in some convoluted way.
They might justify it through her past traumas. For instance, if Sakura has faced repeated rejections or has been constantly overshadowed in the original story, a writer could use this as a reason for her to betray. She could feel that she has been treated unfairly and that her so - called friends never really understood her pain. So, in the fanfiction, she decides to take a different path that is seen as a betrayal to those around her.
A possible justification could be a deep exploration of Harry's post - traumatic stress. Writers might think that after years of fighting and losing so many people, Harry could reach a breaking point. But really, the Harry Potter we know from the books has a strong will and a support system that should prevent such a tragic end. So, these justifications seem more like attempts to create shock value rather than staying true to the character's nature.
Some writers use the idea of a common enemy. If there is a powerful foe that threatens both Artemis and Percy's worlds, they are forced to team up. During this cooperation, they discover new sides of each other. Percy might show his loyalty and courage in a way that makes Artemis fall for him, and thus the relationship is justified.
Some writers justify it by exploring a deep emotional connection. Maybe Draco has been protecting Harry in the fic, and Harry starts to see him as a paternal figure. For example, if Draco saves Harry from a great danger multiple times, Harry might develop a sense of dependence and respect that could lead to this. Another way is through a magical bond. If there's a spell or a magical accident that links them in a way that Harry feels a child - like connection to Draco, it can be used as a justification.
Writers often justify their relationship by showing character growth. For example, Draco might have realized the error of his ways after the war and Hermione, being the empathetic person she is, starts to see the good in him. This gradual change in their personalities makes their relationship seem more plausible.