It could be a matter of self - preservation. If his father is involved in something dangerous that might also put Draco at risk, he might choose to eliminate the threat. For example, if his father made a deal with a more powerful dark wizard that would cost Draco his life in the end.
Perhaps Draco is tired of living in his father's shadow. His father has always been a dominant figure in his life, and by killing him, Draco could be trying to assert his own identity.
One possible reason could be his father's strict and cold parenting style. Lucius Malfoy was often very controlling, pushing Draco to uphold the family's pure - blood ideals and reputation, which might have made Draco feel suffocated and resentful. Another reason could be that Draco witnessed his father's dark deeds in the wizarding world, like being involved with Voldemort, and he might have moral objections to such actions.
One possible reason for such a fanfiction could be to explore the extreme psychological state of Draco. Maybe in this fanfic, Draco is pushed to the edge by his father's overbearing nature and the dark deeds of the Malfoy family. It could be a way to show his struggle for freedom from his family's dark legacy.
One possible motivation could be that Draco has changed. He might have seen the error of his ways and decided to do the right thing. Another motivation could be that he has feelings for Hermione, even if they are not fully realized yet. He doesn't want to see her in a bad situation, so he steps in.
Draco could be seeking redemption. He may be tired of being on the wrong side of the war. Saving Hermione, who is a symbol of the good side, would be a significant step in his journey towards redemption. Additionally, he might be influenced by his friendship or alliance with some of the other characters who are on the side of the Order of the Phoenix. They could have convinced him to do the right thing and save Hermione from Bellatrix.
Maybe Draco is jealous of Hermione's intelligence and abilities. In the fanfiction world, he could think that by making her his wife, he can somehow take credit for her achievements or keep her from outshining him. It could also be a power play. Draco might believe that having Hermione under his control through marriage would give him more influence in the wizarding world, especially if she has connections that he wants to use for his own gain.
One motivation could be that Draco has developed feelings for Hermione. So, when he sees Viktor as a rival for her attention or a threat to her safety, he takes action to save her.
In some fanfictions, it could be a case of a powerful curse that forces Harry to act against his will. The curse might be placed on him by a powerful dark wizard who wants to create chaos in the wizarding world. Draco is just an unfortunate victim of this curse as Harry, unable to control himself, kills Draco. This then sets off a chain of events where Harry has to deal with the guilt and the consequences of his actions.
Maybe Draco discovers a powerful magical artifact that can defeat Voldemort and he decides to use it for the greater good, even if it means going against his family's past allegiances. He could then face many challenges and battles before finally being able to kill Voldemort.
Maybe in some fanfictions, Draco is driven by a sense of self - preservation. He might believe that Harry will eventually turn against him or the Slytherin house. He could think that by killing Harry, he can secure his own place at Hogwarts and in the wizarding community. Additionally, it could be part of a larger plot by Voldemort or his followers. If Draco is forced to join Voldemort's side, he may be given the task of killing Harry to prove his loyalty.
Maybe Gaara was driven by deep-seated anger and trauma. The consequence could be a total change in his relationship with Sakura and his place in the community.