I wonder, why Rachel, though?
Dont get me wrong, under the context of her mission, it fit very well into the story. Also, her deatch scene was written incredibly well, her final thoughts really did sound like those of a dying person, I just don't know why.
I still think Cassie should've been the one to die, I'm not saying that because I hate her (she's my second favorite character), but because I think it would've been more tragic to have the "purest" one destroyed by war. As a matter of fact, at the end, she seemed to come out the best from it.
I, personally, think she picked Rachel because she had the most connections with people out of the Animorphs, and would be missed the most (she had Cassie, Jake, Tobias, Sarah, Jordan, Mom, Dad).