But not everyone is affected by it so drastically. Rachel was made out to be a fiend, which I refuse to accept after the fine character development in the first half of the series. She could be normal again. Marco may be bored, but he's not forever altered to the point where hes not functional. Jake I can't see being entirely functional, because he was the one ultimately deciding who lived and died. Tobias couldn't live with himself before the war and was always mechanical during the war, very I'll do this, and follow this and save them. He would have been better off after the war because at least people respected him. Cassie was just perfect at everything.
On the flipside, I would think it to be very easy to be affected so drastically by something as dramatic as an intergalactic war. Rachel is made out to be a fiend at certain points, and admittedly jumped in and out of character when the ghostwriters were presenting her, but I have absolutely no problem seeing someone like her- vivacious and belligerent as she was from the beginning, transformed into someone more war-driven and ruthless by the end. I believe it would be more out of frustration than anything else- I think Rachel was so caught up in fighting and winning this war because she had so much to protect (family, friends, humanity), that it changed her.
So perhaps it isn't the circumstance that changes a person, but rather that person's reaction to the circumstance so that they in turn change themselves. Reaction out of necessity- the need to adapt to survive.
It is pointed out numerous times that Jake has mentally aged considerably since the beginning of the war. That is a drastic change, and one very obviously pointed out by the author. Such a change would be so apparent in his character anyway that I can only assume she points this out so as to invite her readers to examine the changes in others. That is character development, which KA presents beautifully.
I think the point the author(s) were trying to across was that Rachel was war-driven, excited by the adventure, and so energetic, that the thought of living a normal life would become boring for her. Or, the way I favor to look at it, she developed a slight twinge of insanity. Easily an acceptable fate for her. I do love that she died, as horrible as it is. I do also think her death could have been more necessary, but KA killed off characters who would not or could not have progressed further with their lives beyond the war. I trust her judgement on that- she knows her characters best, as she wrote them.
And I must argue that Cassie was not perfect at everything. Cassie played the part of the introvert- and she played that part well. Struggling with the same difficulties someone like her would go through. There was a lot of mistrust directed at Cassie throughout the entire series, between her decision to help Aftran, to her pointing out reasons why the Yeerks deserved a better life, up to when she allows Tom to take the morphing cube. Cassie makes a lot of necessary mistakes (even though the last is arguably not so much a mistake as it is a flash of instinct, but I'll let that one slide) in order to win the war. She was the one who refused to kill or get involved if she believe the mission was immoral- remaining the most peaceful and thus most unaffected member of the Animorphs. Her ability to see that level of morality at such a young age makes her more immune to the effects of war than it would for the others.
As for Marco, while a bounce back would be possible for him, as I explained earlier, I would think he would be too bored without the fighting. When you live with that level of purpose, going back to a "normal life" would be depressing.
May I point out another popular book where the main characters remained mentally wounded after the war- as I do believe the is why Frodo, Bilbo, and Gandolf set sail to the Undying Lands at the end of LOTR? And while Frodo and Bilbo could surely have only been affected by the ring, Gandolf travels with them as well, suggesting that it is not only the power of the ring, but as the power and affects of war that sets them apart from the others.
Most of it has to do with stable personalities as well, but I won't get in to all that. I feel like I've gone on long enough to make my point. =]