1. I kind of have mixed feelings about that. One half of me says, if you can't handle fighting in the war, there's nothing wrong with leaving it. If you can't handle it, and you're going to drag the team down, it is best to get out. But what she said when she left bugged me and really rang as just self-righteousness and taunting the others with their ability to do what had to be done. It would have been more sincere/tolerable if she had some sort of mental meltdown and just couldn't handle killing anymore, with someone like Cassie it's bound to happen. But her throwing it in everyone's faces that they were becoming monsters was just really arrogant, and not even a 'thanks for saving the world, btw.' I think that the main reason, in this book, that made Cassie drop from my favorite character to my least favorite character. She's so stuck in her moral mentality that she can't see the huge picture, the fact that they essentially have the whole world to think about now. Granted, some of the other Animorphs, particularly Marco, were kind of cruel about the departure.
2. I really liked that about this book, I think that is what really sets this series apart from most series/books. It broke the series from the moral 'black and white' and made readers realize that Yeerks are people too. It added depth to the series.
3. In a relatively peaceful setting, I think anyone can be friends, even polar opposites. I think each of the characters (Cassie and Rachel) had things that the other respected, perhaps Cassie at one point liked Rachel's gung-ho attitude, but it took a war for Cassie to realize just how far that attitude went. I think despite their differences, what tied them together was how much they cared for each other, they just cared for each other in different ways. Cassie nurtured while Rachel fought to save her and the world no matter the cost. When Cassie left, I think the way in which Cassie left really hurt Rachel, because Cassie just couldn't see how much Rachel cared for her, and it was a real slap in the face to Rachel, which was why Rachel said they couldn't be friends. Yet it was apparent Rachel still cared for Cassie a lot.
5. I think Yeerks are stuck between a rock and hard place. As their natural form, they really can't be free in any way, whether they are stuck in their pool or in a host. They still have to leave their host to feed, they still have to (usually) forcibly capture an unwilling creature to experience any sort of freedom. Honestly, I think being free is just being happy in the situation you are in. There will always be a degree of 'unfreedom' in our lives, whether it's government, a relationship, or biology/lifespan. The only way to really be 'free' is to just be happy despite limitations.
7. I don't think the Yeerks have the 'right' to take over anything, which goes back to the rock and hard spot above. If I were a Yeerk, giving the things I knew, and seeing no other way out, I probably would have just done what they were doing. It's hard to say what I'd do, completely, as a Yeerk, but knowing more than what a Yeerk would know, I would hope I would try to find some other solution, like morphing power to get myself out of my Yeerk body, but since relations with Andalites were worse than hostile, I can see that the chances of that would be slim to none, so fighting for hosts would probably be the way to go.
8.I read it a few months ago, I'm not expecting any changes. Without any sort of massive change to Cassie's character/motivations, I really don't think any changes would work. But one thing I think should be addressed is just how Karen managed to fly under the radar after she was released. She had a HUGE part to do with the Yeerks plan...exactly how did they not notice their main source of information on an important resource (her father's money) was gone?
9. I think that Cassie's being able to morph from a butterfly to human, and choosing to do so, kind of highlights the hypocrisy that Cassie displays throughout the whole series. Basically, she broke her pact with Aftran by becoming human again, and she never gives it any more thought, yet she still seems to think she took the moral high-road. There was no self-introspection as to the fact that she broke the pact, that she is now free even though they mutually agreed to give up their freedom. Cassie can't see her own moral failings, literally just blind to them, and I think that's a theme that follows her throughout the whole series. (Though, it was thoroughly reinforced by KAA herself.)