David is one of my worst nightmares generated by this series. I would gladly skip the David series if I'm to reread Animorphs 1-54.
He's bad for one's guilt - yes, he's a human being and he has the right to live, but he's such a pain in the ass that I was really happy that they got rid of him. However, I don't like the Animorphs' decision to let him stay as a rat. They may have picked some animal with a less suicidal life like dolphin, dog or even cat. A person may be bad, but that doesn't mean that s/he should be punished harshly, right?