Perhaps they refused to give Harry access to his family's vault when he really needed it. This could be because of some bureaucratic red tape or because they were trying to hide something in the vault. Such an action would make Harry angry and motivated to get back at them.
One possible motive could be to stop Voldemort from getting his hands on some crucial dark magic item stored in Gringotts. Harry might think that by destroying it, he can prevent Voldemort from getting more power.
One way could be by using magic to disrupt their daily operations. For example, he could cast spells that make the goblin tellers' counting devices malfunction, causing chaos in the bank. Another way might be to expose some of the bank's secrets that they've been keeping from the wizarding community, like hidden vaults that are being used for illegal activities.
It could also be a strategic move. Gringotts is a highly secure place. By buying it, Harry could ensure the safety of important magical artifacts or secrets. He could use it as a base to store and protect things that are crucial for the future of the wizarding world, like ancient spells or prophecies.
Maybe Harry gets assigned to a special project at Gringotts. For example, to help relocate a very dangerous magical item from one vault to another. Along the way, he has to face various challenges like magical traps and other creatures that guard the vaults. This could lead to a series of adventures as he tries to complete the task.
Harry might assist in retrieving lost or stolen items. His bravery and problem - solving abilities could come in handy. If a valuable magical artifact was taken from Gringotts, Harry could lead a quest to get it back. He has a knack for getting into and out of difficult situations, which would be useful in such a task.
Jealousy could be a big motive. Harry is often the center of attention, and Draco, who is used to being in the spotlight in his own Slytherin circle, might be jealous. So he punishes Harry to try and knock him down a peg or two.
One possible motive could be revenge. The Dursleys treated Harry awfully for years, so in fanfiction, he might reach a breaking point. Another could be self - defense if the Dursleys in the fanfiction were to take their abuse to an extreme level.
One consequence could be that he unknowingly steals something that has a powerful and dangerous magic attached to it. This could put him in great danger as he may not be able to control or understand the magic. It could also lead to a chain of events that disrupt the balance of the wizarding world, as things from Gringotts often have a long - standing significance and influence.
One possible motive could be misinformation. If Harry is fed false information about Sirius being a traitor or a danger, he might act on it. For instance, if a well - disguised Death Eater manages to convince Harry that Sirius has joined Voldemort, Harry, in his haste to protect others, could attack Sirius.
Well, in this sort of fanfiction, Ginny's motive could be a complex web of emotions. She might be under the influence of a dark spell. Or perhaps she has been misinformed about Harry's role and believes that by giving him to Voldemort, she's actually saving the wizarding world. It could also be a case of extreme jealousy, if the writer wants to explore a really twisted version of her character.