See, I think that one of the biggest things with David was that he just didn't have a concept of his new reality.
Yes, it was evil for him to betray the human race. But I don't think he saw it that way. I think, in his mind, he saw it of him wiping his hands of the situation and using his powers to advance himself. I don't think he really made the connection that he was potentially dooming the planet. That's a big idea to wrap your brain around.
Similarly, by morphing Saddler, I don't think he saw it as 'evil'. In his mind it was his chance at regaining a life for himself, especially since there was so little chance that the real Saddler would make it.
That's not to say that what he did wasn't evil. But I think it's important to note that David didn't believe he was being evil. He was just the kind of person who only looked out for himself. It doesn't excuse his actions, but perhaps it explains them a bit.
Exactly! I agree with this. He wanted to bargain with the Yeerks, trade the Escafil for his parents. He wanted to rebuild the life that had been taken away from him. He thought if he fought beside the Animorphs, he would die. (Not a bad assumption. A few books later, they hire on some more new Animorphs, and look at what happens to them!) He thought if he tried to resign, they'd turn on him. (Could they trust him to just walk away? Somehow, I doubt it.) So he struck preemptively. What he did was terrible, but in his mind, it was fully justified.
Although I disagree that leaving David to the Yeerks would have been the best option... he had memories, after all, of Marco being very eager to buy the Escafil, and that same night Andalites attempting to steal it. David tells Marco that the e-mail with his address goes out right as he gets home, and a few hours later the Andalites are waiting in David's house when Esplin gets there. Coincidentally, this kid is the son of the host body of a Yeerk who saved the Andalite bandits in #5, was saved by an Andalite bandit in #15, and would later be spared by an Andalite bandit in... I think #25.
I'd think it'd be a security risk to let David over to the Side of Evil. They should have killed him then...