It could be to convey a certain moral or lesson through the events of the story. Or perhaps they just have a compelling tale to tell and think others would find it interesting.
People write 'Armin Arlert death fanfic' for various reasons. It could be a form of expressing their own interpretation of the 'Attack on Titan' story. They might feel that in a certain scenario, Armin's death is a logical outcome. Another reason could be to explore the psychological impact on the other characters if he were to die. Writers may also be interested in subverting the expectations of the original story. Since Armin is an important character, his death can change the entire dynamic of the story and open up new avenues for storytelling. Some writers might also use it as a way to deal with their own feelings about the character, either loving him so much that they want to explore a tragic end for him or having a different view of his role in the story.
John Lithgow has a very distinctive voice. It's deep and rich, which can draw children's attention immediately. For example, in his narrations, he can make different characters sound unique just by changing the tone a little bit.