We're all forgetting one very important point. David was a CHILD. Sure, he wasn't a little kid, but he still hadn't reached maturity yet. Like I bring up to people when discussing Elfen Lied, should Kaede be held accountable for murdering Kouta's family despite her being a little girl? Getting back to the topic at hand, it's possible David might have grown out of it. I do agree he would have turned traitor eventually, but I feel that the Animorphs, Marco particularly, are never willing to admit that they messed David up. If they'd committed to him 100%, then they would be less accountable than they are now, with Marco condescending down to him and making him sleep in a freaking barn. I would say David is 75% responsible and the Animorphs are 25% responsible, or somewhere closer to 50-50. Though I do like the twist in the fanfic One Least Likely where David grows a crush on Cassie, and that keeps him from crossing over to the dark side.
To address donut's point, it doesn't matter. Even if they were stopping the Yeerks, those acts were still atrocious. And David hadn't really done anything that evil until he dropped Saddler down the elevator. It was dark and morally ambiguous, but until he took Saddler out, you could argue he really viewed it as him defending himself from the people he hated because, remember, they didn't treat him all that great, again, Marco in particular. Besides, in the case of Cassie, she had the freaking TIME MATRIX! She could have traveled to the exact moment before Visser Four came upon the Time Matrix and stopped him then, even killing him, which is a much better fate than what is essentially destroying his soul. To quote poparena: "Stupid, stupid Cassie!"