Another motive could be to prevent a greater evil. Morgana had the potential to cause even more destruction. If she were to gain more power or form stronger alliances with dark forces, the consequences could be catastrophic. Merlin, with his knowledge of magic and the future of Camelot, may have believed that her death was necessary to safeguard the kingdom and the world at large. He might have weighed the value of her life against the countless lives that would be at risk if she continued on her dark path.
One possible motive could be self - defense. Morgana had turned to the dark side and was constantly causing harm to Camelot and its people. Merlin, as the protector of Camelot, might have seen killing her as the only way to stop her reign of terror. For example, she was plotting against Arthur and trying to bring down the kingdom. Merlin's loyalty to Arthur and Camelot could have driven him to this extreme decision.
It could also be a result of some external influence, like a dark magic spell that has clouded Ron's judgment. This spell could be making him act against his normal character and do something as extreme as kidnapping Hermione. However, in the end, he would likely break free from this influence and realize the error of his ways.
One possible motive could be extreme guilt. In the Star Wars universe, Luke has faced many challenges and made some tough decisions. If he were to make a huge mistake, like accidentally leading the dark side to a major victory, the guilt might be overwhelming. He's a character with a strong sense of justice and responsibility, and such a blunder could make him feel unworthy of living.
One possibility is that there would be a great deal of tension. Old Merlin has seen so much and has his own wisdom, while Morgana has her own powers and motives. Maybe Merlin would try to persuade Morgana to abandon her dark path. He could use his experience and the bond they once had to appeal to her better nature.
One possible motive could be to protect Sasuke. Itachi might have seen a great danger approaching Sasuke that he could only avoid by taking him away forcefully. Maybe there was a powerful enemy targeting Sasuke, and Itachi thought that by kidnapping him, he could keep him safe in a hidden location and train him to become stronger.
One possible motive could be a conflict over their loyalties. Remus is a werewolf and has his own set of insecurities and allegiances. Maybe Snape discovers something about Remus that he threatens to expose, leading Remus to kill him in a moment of panic. For example, if Snape were to find out about Remus endangering students during his werewolf transformations and was about to report him to the authorities.
Guinevere would also be impacted. She was a part of the Camelot court and knew both Merlin and Morgana. Her kind heart would likely be troubled by the violence and the loss of Morgana, even if Morgana had become an enemy. She might try to mediate between Arthur and Merlin if their relationship became strained. And she would have to help Arthur through the difficult time of mourning his sister, while also dealing with the implications of Merlin's actions.
Well, it could be a culmination of events. Uther's growing distrust of those around him, combined with Merlin's secretive nature about his magic. Merlin might be caught in the middle of a power struggle or a plot against Uther, and Uther, in his quest to protect his throne and Camelot, decides to eliminate Merlin without fully understanding the consequences. This would also create a great deal of drama and conflict for the other characters in the story, which is something fanfiction writers often strive for.
Merlin would likely become more cautious. In a time - travel situation, he'd know that his actions could have huge consequences for the future. For example, if he accidentally reveals too much about his magic in the past, it could change everything. Morgana, on the other hand, might see an opportunity for power. She could try to use her knowledge of the future to gain a stronger position in the past.
One possible motive could be a complete breakdown of the peace they had worked so hard to build. Maybe an outside force manipulates Gaara's mind, causing him to turn on Naruto. Another could be extreme jealousy over Naruto's popularity or power.
One possible motive could be extreme jealousy. Maybe in this fanfiction, Hinata is insanely jealous of Sakura's relationship with Naruto or her skills, and that drives her to the unthinkable act of killing.