In a way, she kind of took responsibility on herself for what they had to do. Contrary to popular opinion, danger isn't the source of PTSD and similar psychological phenomena. Rather, the trauma of deliberately killing other people is a major stumbling block, especially when there is no personal reason to want their deaths. That's why they blindfold or hood people who are being executed, so those carrying it out don't have to look them in the eye. It's also why they use firing squads instead of one guy with a gun. The ability to mentally place the blame or responsibility for even a righteous killing is a tremendous alleviation of the psychological weight of taking a life. It's tremendously easier to go into a hand-to-hand battle when someone else is saying "Let's do it!" Now, you're just going along with the flow, because crazy old Rachel would be doing the killing anyway, so you're just part of the machine. It's not actually YOUR fault that strangers are getting eviscerated.

But there was no one to take that responsibility off Rachel's hands. The most psychologically boned of the Animorphs after the war was Jake, because he had the biggest burden of responsibility. Cassie and Marco were far more adverse to violence, and should have been much more shaken up why what they had to do, but they were more or less okay, because Jake was giving the orders. What messed up Marco the most was not necessarily the idea of losing his mother, but the time when he initiated and carried out a plan to assassinate Visser One, in which she would be collateral damage. Because he was responsible.

By Rachel's last couple of books (specifically the return of David, and the last book), she was clearly on the edge, and beating herself up, despite having never actually committed an immoral murder. She was explicitly delineating herself as the one who took responsibility for the violence and the more brutal necessities of their war. Pretty the same speech Gordon gave at the end of "The Dark Knight" could have been said about Rachel.

Hope this helped you reminisce.