Sorry, but none of it made sense. Wasn't entirely well thought out, now was it. But hey, even writers make mistakes. It happens.
I'm sorry for bumping this topic but I just have to point this out...
It wasn't a mistake. It was war. A main motif of the story, war is not pretty and you do what you have to in order to ensure a victory. Didn't you listen to Jake's speech before the plan was in place? He said half of them would die and that's okay, because the end more then justifies the means. It was a war, it was either the Yeerks or the Humans, Jake's species, Rachel's species, the Original animorph's species, the Auxiliary Animorph's species, Jake knew what he was getting them into and he accepted it. Rachel was more then accepting to sacrifice her life for the greater good, as long as it meant the end of the war. After she stated that the polar bear had raised it's paw and killed her with one swipe, the Ellimist appeared, all Rachel wanted to know was if her life and death mattered and it did. That's that.
Did she have to die? No, not exactly but she did and that's what happened.
It was also kind of symbolic, considering everyone had wondered at one point or another what she would really do once the war was over. Even Tobias. Well, we never found out and maybe that's a good thing.