Stephen King writes adult horror movies and junk, right? I wouldn't care if a random dog died in some book, just like I don't care that much that the auxilary animorphs all died. I think it was wasted potential, but beyond that I don't really care.
Rachel isn't even one of my favorite Animorphs, but I felt for her enough that I hated that she died. Of course, Rachel dying is just part of what made the ending really bad.