Their relationship would likely become more intense. They would have a new bond through the baby. Edward would be even more protective of Bella, not just because he loves her, but also because she's carrying their child.
One possible plot could be that they face various challenges during the pregnancy. Maybe there are threats from the vampire world that don't approve of their union creating a new life. The baby could have some unique powers that both Edward and Bella need to figure out how to handle.
Well, it might become stronger as they face this new challenge together. They have to rely on each other more for support and decision - making regarding their baby.
One unique feature is the exploration of their new roles as parents. Bella and Edward have to deal with the normal human aspects of parenting like feeding and diapering, while also considering their vampire - human hybrid baby's special needs.
I'm not aware of any official 'Edward Kills Bella' baby fanfiction. In the original 'Twilight' series, their relationship is complex but not of this extreme nature. Fanfiction can vary widely in themes, but this seems like a very dark and uncharacteristic take compared to the source material.
Another reason might be Edward's overprotectiveness. He could have made a decision for Bella without consulting her first, like preventing her from seeing a friend. This kind of controlling behavior could make Bella feel frustrated and end up in tears.
One challenge could be financial. Raising a baby is expensive. Another might be dealing with family opinions on how to raise the child. And there's also the physical exhaustion from taking care of a baby.
It could be the Volturi. Since the baby is a vampire - human hybrid, the Volturi, who are very strict about vampire laws, might see the baby as a violation of their rules and try to eliminate it.
Well, one possibility could be that the baby has some special powers right from the start. Maybe it can communicate with the vampires in a unique way that no other vampire - human hybrid has before.
One common theme is the challenge of growing up quickly. Since they are teens having a baby, they have to take on adult responsibilities much sooner than expected. Another theme is the strength of their relationship. Their love for each other is tested and often deepens as they face the difficulties together.