I have mentioned this in other threads, but I have yet to make a thread specifically about this. Cassie let Tom get away with the Escafil Device, and I can't fathom why some fans would not hold that against her.
There were good things that came from it. However, any possitive thing that would come from that, could have been done while being done differently. If they had used the Chee, or PM Yeerks, to spread the word that the Animorphs were willing to give a way out for Yeerks who didn't want to be involed in war or involuntary infestation, then there would be Yeerks coming to become nothlits. It would make a positive effect, without the downside of Visser Three having the device, or being worried about squads of morph capable controllers attacking the Animorphs.
In the moment when Cassie decided to let Tom go, she was afraid that if Jake killed Tom he would be changed. However, he already wasn't the same Jake she once knew. Plus he wound up killing Tom anyway. Of course, this was to make sure he didn't become another Eva. Better dead than a controller: The Ani motto.
Some say that it was Applegate's way of making hope seem as lost as possible, but even so that would still count as a dumb move by Cassie. Nothing good came from doing that, that couldn't have been gained some other way.
One thing that bothers me, is that when Jake excludes Cassie from a war meeting, Tobias says that's "beyond wrong". I very much disagree with this. I'm glad the group was willing to forgive her for that, but when someone breaks another's trust, it's a big issue.
Don't get me wrong, though. I don't hate Cassie, and I'm glad Jake still had feelings for her. It's good that their relationship is strong enough to overcome a large mistake.
I do hold it against her. Some people don't, and that confuses me. She could have helped Jake take Tom down. Between the two of them, they could have subdued a Human Controller without killing him, even though he had a dracon beam. Some say there may have been enemies in the area, but I don't think there were. I think most everyone else was back fighting the battle with the other Anis and the auxilaries.
I'd like to speak about this in detail, on topic. Then maybe put it to rest.