Part of what went wrong in 19 is that Cassie didn't seem to understand who Rachel was. Which is a pretty deal for a best friend. Rachel has her dark side, but she also has a strong better side too. She's not just a vicious blood knight, like the others think she is over time. She's definitely one of the strongest Animorphs, and abandoning the world because you don't want to become like Rachel shows a certain amount of ignorance on her part.
Giving the box to the Yeerks, and acting like whatever good came from it was her plan all along.
Having a fit over killing an animal in the second megamorphs book. Hell, getting upset over killing a termite queen too.
I realize that at the start all the Anis are kids. 13 years old. However, Cassie refuses to grow up and accept that people have to change. The world isn't made of butterflies and sprinkles, no matter how much we wish it would be. She's a static character, and while I wouldn't say she's pointless, she is annoying.