One possible reason could be a radical change in Bulma's character. Maybe she's been corrupted by some dark force or has some deep - seated grudge that has been building up over time. But again, this is very uncharacteristic of her normal self.
In the fanfic, it could be that Bulma has fallen out of love with Vegeta. Maybe she has met someone else who seems more compatible with her. This new person could be more in tune with her technological interests or simply be more emotionally available. Or it could be that an event has occurred that has made Bulma see Vegeta in a different, less favorable light. For instance, if Vegeta made a decision during a battle that put their family at risk without consulting her, it could be a breaking point.
One reason could be to create a huge plot twist. It's a very unexpected and controversial idea that can immediately grab the reader's attention. Another reason might be to explore different emotional states. For example, showing how Vegeta would react to such a betrayal and how Goku would deal with the moral implications of being involved in such a situation.
In some fanfics, Bulma might be ignoring Vegeta because she is focused on her own scientific projects. She's always been a brilliant scientist, and if there's a major discovery or problem she's dealing with, she could be so absorbed in it that she has little time for Vegeta. It could also be related to the way Vegeta has behaved. If he's been too overbearing or made some mistakes in their relationship, Bulma might choose to ignore him as a form of punishment or to make him realize his errors.
One possible plot could be centered around how Vegeta adjusts to the idea of becoming a father. He's a proud Saiyan warrior, so he might initially struggle with the more nurturing aspects. For example, he could be shown trying to train while also being worried about Bulma's well - being during her pregnancy. Maybe he starts to research Saiyan pregnancies to make sure everything is going as it should.
I'm not aware of any such fanfic where Bulma murders Vegeta. In the Dragon Ball universe, they are a couple. Fanfics usually explore positive relationships or other adventure - based scenarios for them rather than such a dark and uncharacteristic plot.
One possible plotline is that they are on the rooftop to escape the chaos of the city below. They find peace and quiet there, and it becomes their special place. They might start reminiscing about their adventures together and how far they've come as a couple.
One possible reason could be stress. In fanfic scenarios, she might be under a lot of pressure from her scientific research or protecting the Earth. The stress could build up to a point where she starts to harm herself as an outlet.
In such a fanfic, they might have a deep conversation. Maybe Vegeta could open up about his past and his insecurities. Bulma, being the understanding partner, would listen attentively and offer her support.
Well, in a fanfic like this, perhaps it could explore their relationship deeper. Maybe it shows how their different personalities, Vegeta's tough Saiyan nature and Bulma's feisty human side, come together in an intimate way. It might also involve how they deal with the challenges of being from different species while being in love.
Well, it could be about them being classmates in high school. Maybe Vegeta is the new, mysterious transfer student who catches Bulma's attention. She might be the popular girl with a love for science, and they start off bickering but gradually develop feelings as they work on a school project together.