Well, an 'Evil Morty' fanfic in the 'Rick and Morty' universe can be really interesting. Evil Morty is such a complex character. In these fanfics, he could be explored further in terms of his motives. Maybe he's tired of being in Rick's shadow or he has some grand plan to take over the multiverse that wasn't fully shown in the show. Fanfics give the writers the freedom to expand on his backstory and what makes him tick.
One more thing that makes these fanfics unique is the way they can twist the relationships in the show. Evil Morty's presence can change the dynamics between Rick and Morty. For instance, Morty might start to question Rick more if he sees what an Evil version of himself is capable of. And Rick might have to reevaluate his relationship with Morty in light of this new threat. This creates a whole new set of story possibilities that are unique to these fanfics.
By putting them in situations where they have to rely on each other. For example, if they are trapped in a dangerous dimension. This would force them to interact and potentially grow closer.
Evil Morty's origin likely began with his exposure to the many dark and unfair aspects of the multiverse. He was probably fed up with the Ricks' arrogance and control. Maybe he had some unique experiences that made him question his place. He could have started small, observing how things worked and gradually formulating his plans. It's also possible that a particular event, like being mistreated by a Rick, served as the catalyst for his turn to evil.
Since I don't know the specific 'bad end friends story', it's hard to say who the characters are. But generally, there would likely be at least two friends. They could be friends from school, like classmates who have different personalities. Maybe one is more outgoing and the other is shy, and their differences cause problems in the end.
The 'bad end friends story' might be centered around a series of events that test the friendship. For example, if the friends are involved in a competition, jealousy could rear its ugly head. One friend might feel that the other is getting more attention or opportunities, and this envy could lead to backstabbing or other harmful behaviors. Or it could be a situation where a friend makes a big mistake and the others can't forgive them, resulting in the end of the friendship.
One idea could be that Morty discovers a way to redeem Evil Morty. He tries to show Evil Morty that there's a better way than his villainous path. Morty could use his own experiences of growth and kindness to reach out to Evil Morty.
One idea could be to explore a scenario where friends turn on each other due to a misunderstanding over a valuable heirloom. Maybe one friend was entrusted with it for safekeeping but lost it, and the others think it was stolen on purpose.
One common theme could be betrayal. For example, a friend might betray another for personal gain, like stealing a job opportunity that was promised to the other friend. Another theme could be miscommunication. Friends might misunderstand each other's intentions and end up in a bad situation. Also, jealousy can play a part. If one friend is jealous of the other's success or relationships, it can lead to a bad end in their friendship.