Let's face it. We go to war not only to win it, but to achieve justice and peace.
That being said, most countries treat it like a massacre.
Book #19, The Departure, shows another side to Yeerks, proving that they're not all evil. Yes, Cassie wanted to make a pact with a little girl named Karen and her Yeerk, Aftran. Cassie wanted to make peace. But Cassie is just ... being stupid.
For one, she is an Animorph. Sure, in that book she quit the Animorphs, but trapping herself in caterpillar morph was pointless. That little girl named Karen became a Controller so the Yeerks could get closer to her dad, who owned a huge bank. The Sharing needs to get their funds from somewhere, you know.
And by agreeing to become a caterpillar nothlit if Aftran left Karen, she should have been smart enough to know that Karen would just get reinfested. The Yeerks needed Karen to use her daddy's piggy bank. News reports show that both Karen and Cassie are missing - three days later, both of them are well and alive, and what do you know, Karen doesn't go back to the Yeerk Pool. The Yeerks aren't that stupid. She's a little girl, and a re-infestation would not only give away Aftran's knowledge of Yeerks who hate their parasitic lifestyle but EXPOSE THE ANIMORPHS IDENTITIES as well. Thus, Cassie is stupid. Very stupid.
And sure, she has her morals and ethics, but in a war, they don't always work.
In book 50, the Ultimate, she stopped Jake from killing Tom. Honorable of her. I would have been glad had she done that if Tom DID NOT HAVE THE MORPHING CUBE and thus let the Yeerks acquire morphing power. Morphing was the one thing they had against the Yeerks and she delivered the weapon straight to them. The only "bright" side to her action was that Tom survived...only to be killed by Rachel later on Jake's orders.
Sorry for rambling, but as moral and ethical as Cassie is, she's not really cut out for the war.