It's possible that Snape was forced to do it. Maybe some dark force or an evil wizard had control over him and made him turn Harry into a vampire as part of a larger, more sinister plan.
Snape could be motivated by some dark magic experiment. He might have thought that turning Harry into a vampire could unlock some hidden power that could be used against Voldemort or for his own gain in the wizarding world politics.
In fanfiction, Snape might be begging Harry to marry him because he has a newfound respect for Harry's strength and heroism. Snape, who has always been a complex character, could have had a revelation about Harry's true nature. He may also be seeking redemption through this extreme act. By tying himself to Harry in marriage, he could be trying to rewrite the wrongs of the past. It could also be a result of some sort of magical bond or curse that forces him to have these feelings. Or perhaps he's just lost his mind in a really wild fanfiction plot twist.
I'm not sure there are a lot of such specific fanfictions, but you could start by looking on major fanfiction platforms like Archive of Our Own. Try searching for the exact phrase 'Harry turned vampire Snape mate' in the search bar. There might be some hidden gems there.
Snape might curse James out of jealousy. In many fanfictions, Snape is jealous of James' popularity and his relationship with Lily. So, his curses could be a way to get back at James for having what he wants.
One consequence could be that Harry is ostracized by some in the wizarding community who fear or misunderstand shifters. This would isolate him and make his life more difficult.
Perhaps Harry sees a bit of himself in Atreus. Atreus is on a journey of self - discovery and growth, much like Harry was during his years at Hogwarts. Harry could be drawn to Atreus' courage and determination, and so he decides to save him when he's in trouble. It could also be that Harry is just exploring this new and strange world of God of War and when he comes across Atreus, he feels compelled to protect him as he would any other innocent in his own world.
Well, if Snape turns Harry into a vampire in fanfiction, it would completely change the dynamic. Harry would have to deal with new instincts like the thirst for blood. His relationships would be strained as his friends might be scared of him. Also, his role as the 'Chosen One' would be redefined as he's now a different kind of being with different needs and abilities. He might have to struggle between his human side and his new vampire nature.
One motivation could be revenge. If Harry witnessed more heinous crimes committed by the Death Eaters or other dark wizards, he might decide that the normal ways of dealing with them through the Ministry of Magic are too slow or ineffective. He could take matters into his own hands and become an assassin to quickly dispense justice. Another motivation could be a sense of powerlessness. If he feels that he's constantly being manipulated by Dumbledore or other powerful wizards, he might turn to assassination as a way to take control of his own destiny.
She could be frustrated with the Ministry of Magic in Britain. Maybe they're not taking Voldemort seriously enough or are even hindering her attempts to fight him. So, she decides to go directly to Voldemort to end it all on her own terms.
One possible motivation could be the unjust treatment of certain magical creatures. In the wizarding world, there are many species like house - elves that are often exploited. Harry, having a strong sense of justice, might form a rebel group to fight for their rights. He has always been empathetic towards the underdogs. Another motivation could be the corruption within the Ministry of Magic. We've seen how easily the Ministry can be swayed and corrupted, as in the case of Dolores Umbridge's reign. Harry, with his experience of facing such corrupt forces, would want to create a group to oppose this and bring about real change in the wizarding government.
One common theme is the forbidden love aspect, as Snape and Harry have a complex relationship in the original series. Another could be the power dynamics, especially with Snape being a vampire and how that affects his relationship with Harry.