He could be a bit confused at first. Maybe he doesn't fully understand why Bella is so scared. But then he would likely start to take steps to ease her fears. He might open up to her more about their relationship and what it means, so that she feels more in control and less frightened.
Well, perhaps she's scared because she doesn't fully understand what it means to be Sam's mate. There could be some sort of mystery or unknown elements associated with this bond. It might also be that she's afraid of the potential danger that Sam's world or position might bring into her life. For example, if Sam has enemies, she could be worried about being targeted.
There could also be some human characters in the fanfiction who are friends with Bella. These characters might not understand her relationship with Sam at first and could cause some drama. They might try to pull Bella back into the normal human world, while she's being pulled towards Sam and the werewolf world.
She might get angry and defensive. In many fanfictions, characters don't like to be insulted. So, Bella could lash out at the person who called her a brat, demanding an explanation.
Bella might initially be very hurt and confused. She may try to understand why her mother is being this way and might even blame herself at first. For example, she could think that she has done something wrong to deserve such treatment.
On the other hand, she could see it as an opportunity for a new start. She might gather her strength and decide to prove herself outside of the pack, or she could try to find a new group or community where she would be more accepted.
She could be heartbroken. After all, she's pregnant and the person she loves has left her. She might feel abandoned and alone, crying a lot and wondering why Edward would do such a thing.
Often, Bella reacts with intense heartbreak. She might isolate herself from her friends and family, feeling like she can't trust anyone anymore after Edward's betrayal.
Bella might initially be terrified. She is a human being kidnapped by a vampire, after all. She could struggle and try to escape, using her wits and any available resources. However, since she has feelings for Edward in the original story, part of her might also be conflicted, wondering if there is a good reason behind his actions.
One possible fanfiction could be that Bella is caught in a sudden and violent snow storm while on a hike in the mountains. She's scared as she realizes she's lost her way and her phone has no signal. The cold is seeping into her bones and she starts to hallucinate about strange creatures in the snow. But then she remembers some survival skills she learned before and starts to build a snow shelter to wait out the storm.
One key element is the exploration of a different relationship dynamic. Instead of the traditional pairings in the original story, Sam and Bella's connection brings new angles to the story. Another element is the way their relationship affects the surrounding characters. For example, how the werewolf pack reacts to Sam having Bella as his mate.