No Rachel didn't have to die. I think the ending of 48, and one of her last thoughts being about shopping showed she was not just a psychopath. Besides, if she really wanted action she could have found plenty of it. It's not like winning the war put an end to all conflict on the planet. She would have found something. She could have lived a normal life, and killing her off at the end was really bad in my opinion. Yes I wanted a happier ending.
Most of the characters got what they had originally wanted in the end. All except Jake. Marco got to save his mother, Rachel got to save Humanity, Cassie got to save the world, Tobias got to do something important with his life, and Ax stepped out of his brother's shadow. So not only did Jake not get what he wanted, even when he was starting to move on with his life KA decided to send him off to possibly die in outer space.
A lot of people who defend the ending throw up a strawman, thinking the ending has to be either one extreme or another. It doesn't. We just expected it to end like just about every other book. The Anis went through hardships and sometimes loss, but at the end of the day they were usually well enough off afterwards. It didn't have to be a magically happy ending where all was well, but it could have been a better ending where they all grew and took something good from it. Yes war is hard, but they had decades worth of life to cope with the stress, go to a bit of therapy, and still be fine.
I will never agree that Rachel needed to die. If the authors really wanted me to believe Rachel was lost, they should have had her accept Crayak's offer. But she didn't, and she needed no outside encouragement or pleading to do it. She came to that conclusion by herself, and it gave me assurance that she would be fine. Yes she was the darkest member, but she wasn't a total blood knight.
Lastly, the only time I don't expect to see a happy ending, or want one, is a horror story, or a nonfiction story where things just didn't go right. Not everyone who goes through war comes out horribly scarred and unable to contribute to society anymore.
By the way, welcome to the forums.