Obviously, Edward would be deeply impacted. He has dedicated himself to Bella, and her new evil nature would force him to reevaluate their relationship. Jacob, who has always been in the mix with Bella, would face a great deal of turmoil. And don't forget Carlisle and the rest of the Cullen family. They have accepted Bella into their family, and her going evil would be a major threat to their way of life and their values.
Edward would be at the forefront of those affected. His world revolves around Bella, and her evil turn would shatter his perception of her. Jacob would also bear the brunt. He has his own feelings for Bella, and this change would be very hard for him to accept. Then there are the Volturi. If Bella goes evil and starts causing trouble, they would likely get involved as they are the ones who enforce the vampire laws, and her actions could disrupt the vampire world order.
In this type of fanfiction, the theme of identity crisis often pops up. Bella's transformation into an evil character makes her question who she really is. Additionally, themes related to moral ambiguity are common. Since she's going evil, the lines between right and wrong become blurred for her. And the theme of a new purpose can be there too. Maybe her evil self has a different goal, like ruling over a certain group, which is different from her previous goals as a'sweet' Bella.
Bella, obviously. She is the one who is pregnant and will have to face all the physical and emotional changes that come with it. Her life will be completely turned around as she prepares to be a mother.
First off, Edward is likely to be deeply affected. He has spent so much time and energy trying to protect Bella and keep her safe. Now, with her humanity off, he may feel like he has failed. Jacob, too, will be affected. His feelings for Bella, whether it's love or friendship, will be challenged. Also, Bella's father would be affected. He has always cared for his daughter, and seeing her change so drastically would be heart - breaking for him.
The Volturi are frequently in 'evil bella fanfiction'. Since Bella's actions, if evil, would likely draw their attention. They would be the ones trying to maintain order in the vampire world and would see Bella as a threat. Alice might also be involved as she can see the future and would be trying to predict Bella's next moves.
The Hidden Leaf Village itself could be an antagonist. If Naruto goes evil, the village that once was his home would be against him now.
Well, it could be the people she targets. They would be important as they drive the action when Bella is on her 'killer missions'.
Edward could be one of the characters. He might be in a love - hate relationship with Bella due to her addiction. He could be torn between his love for her and his inability to stand her self - destructive behavior.
Foster parents are likely to be important characters. They could be kind and nurturing, or perhaps they have their own issues that create more drama for Bella.
She'll probably meet Stefan and Damon Salvatore. They are central characters in Mystic Falls.
The Cullens are very likely to be involved. Edward, of course, would have a huge role as Bella's former love interest. Carlisle, with his knowledge and experience as a vampire, could help Bella adjust to her new life. Alice might use her precognitive abilities to foresee any problems Bella could face. Also, Jacob could be in the story. His reaction to Bella becoming a vampire would be interesting, considering his past relationship with her and his werewolf nature.