Love and acceptance is also a prevalent theme. Since Beckett comes into Castle's life and becomes part of Alexis's world, fanfics may focus on how they all learn to love and accept each other fully. This could involve storylines where there are initial misunderstandings but eventually they come to a place of complete acceptance and harmony in their relationships.
There are definitely some great 'castle beckett and alexis fanfic' pieces. If you like stories that explore the emotional side, you might find some where Alexis has to deal with the dangerous aspects of her father's and Beckett's job. Others could be more light - hearted, showing their day - to - day interactions in a humorous way.
She could be defensive of Beckett. Since Alexis probably has a good relationship with Beckett, she may try to convince Castle that he's wrong and that Beckett didn't mean any harm. For example, she might tell Castle about all the positive things Beckett has done for her.
You could also check out FanFiction.net. There are fanfics that might take a more adventurous take on the Castle - Beckett - Alexis dynamic. For example, they could be involved in a mystery together where Alexis plays a more crucial role than in the original show, adding new layers to their relationships.
It could be that Castle is overprotective. He loves Alexis so much that even the slightest sign of Beckett not being on the same page as Alexis in Castle's eyes makes him think Beckett hurt her. Maybe Beckett gave Alexis some career advice that Castle thought was too pushy.
Alexis might confront Beckett directly. She could say something like 'I think you're cheating on me. What's going on?' This is a very straightforward approach.
A Castle and Beckett baby fanfic could be about their journey into parenthood. It might explore how they adjust to having a baby in their lives. For example, Castle could be super excited and start writing new stories inspired by his new family experience, while Beckett might be a bit more cautious and protective.
A common theme is friendship. Many fanfics explore how Beckett and Alexis become friends and the bond they share. Another theme is growth, as they both develop as characters within the fan - created stories.
Well, without having read the actual 'beckett and alexis killed fanfic', it's difficult to give a detailed account. However, generally speaking, it might be a piece of fan - made fiction that focuses on a plot where Beckett and Alexis are somehow related to a killing. Maybe they are the ones who do the killing, or perhaps they are witnesses or victims in a more complex scenario.