It could also be a plot device to create drama. If Clark is acting like an ass, it can lead to all kinds of conflicts and emotional turmoil in the story. It might be a way to test Supergirl's character, to see how she reacts to someone she once respected or looked up to behaving badly. Additionally, the writer might be trying to explore power dynamics. If Clark, who is usually seen as a powerful and moral figure, starts acting in a negative way, it can shift the power balance in the Supergirl - centric world of the fanfic.
Perhaps the writer has a unique view on the Superman/Supergirl relationship. They might think that in a certain situation, Clark would become jealous or overprotective in a negative way towards Supergirl. This could be due to differences in their ideologies or the way they use their powers. Or it could be a simple case of the writer wanting to subvert the traditional, positive image of Clark for the sake of creating a more complex and unpredictable story.
Finding a fanfic with such a specific and somewhat unflattering description might be a bit tricky. You could also check out fanfic communities on Reddit dedicated to Supergirl. Post a request there asking for fanfics where Clark is an 'ass' in the context of Supergirl. There are some very knowledgeable fans in those communities who might be able to point you in the right direction.
Maybe it's Supergirl's superpowers. The ability to fly, super strength, and heat vision are all really cool. Bella might be attracted to that sense of power and freedom that Supergirl has.
Maybe he realizes that the shinobi world is too corrupt. There could be political intrigue and power struggles that go against his ideals. He has always fought for peace and unity, but if he sees that the system is beyond repair, he might choose to quit. Also, he could have a family situation that requires him to step away from the shinobi life. For instance, if he has a child with special needs that demands his full attention.
One reason could be Naruto's sheer determination. He has always been the one with an unwavering will, and in this fanfic, that might be the key factor. His never - give - up attitude gives him the extra boost to defeat Sasuke.
Maybe she's injured. In many fanfics, characters can get hurt in various ways, like a fall from a horse or an accident, which could lead to her being unable to move.
Maybe he was blamed for some trouble he didn't actually cause. For example, if something went missing in the orphanage and the caretakers wrongly thought Naruto took it.
One possible reason could be some kind of dark magic or jutsu that has affected his family's memories. Maybe an enemy cast a spell specifically to make his family forget about him to weaken him.
It could also be for the sake of creating a new and unexpected storyline. Fans are used to Percy being the one in danger or getting into trouble. Switching it to Annabeth grabs the readers' attention and makes them wonder how she'll fare in a role that is typically Percy's. It allows for new character development and different relationships to be explored within the fanfic world.
One possible motive could be that Clark has been mind - controlled. Maybe some powerful villain has gotten into his head and is making him act against his will. Another reason could be a misunderstanding. Clark might think Kara has done something extremely dangerous or wrong that endangers the world, so in his warped sense of protecting the planet, he tries to kill her.
One possible reason could be that he's framed for a crime he didn't commit. Maybe someone powerful wants to get rid of him and plants false evidence. For example, they could make it seem like he attacked a village ally when in fact he was defending it from a hidden enemy.