I'd say my main problem with David was that I didn't consider him to be . . . well, very smart, but the Animorphs apparently did. He seemed to have a certain mental block barring him from understanding a few basic concepts, like for example being an Animorph was not at all like being in school, he keeps coming back to the Animorphs being some sort of clique and how he's the new guy that nobody likes and it gets really irritating after a while, he actually uses that nonsense hand in hand with Tobias not being a human to justify murdering him, many of the things he says just show me an insecure kid who tries to be cool by owning a cobra which is illegal, but telling people about it, is clearly jealous of characters like Rachel and just really isn't capable of grasping the situation he's in, and that kind of stuff can be forgiven except that killing someone over that bull is crossing a major line, feeling left out, jealous, and confused doesnt equal a right to take human life even if it isn't in human form David did this or at least thought he did it once and kept trying to do it again.
I do think however that the Animorphs made one major mistake that made David's choices seem more limited to him, see with Jake he just knew they'd try to rescue Tom, Marco's initial reason for remaining with the group was only to help Jake, and with Marco's mother Marco knew Jake would never let her stay a controller and Jake said as much, see nobody even once suggested to David that they'd rescue his parents, did he even know that was on the table? Did they even think of it? I don't remember them ever even saying that they'd try, and why not? Sure Visser Three would be expecting it at first, but if you're going to be so ambitious as to plan on rescuing Visser One who isn't even on earth most of the time the least you can do is tell David you'll try to rescue his parents, give the kid some incentive, instead he had none.
David didn't really have anything to fight for, what was in it for him? Continued survival? That was arguable because the missions could get him killed, so what else? Friendship? He was too closed off to let the others even try to be his friends, and lets face it, kid was kind of a jerk anyway. The only perk they told him about was turning into animals, but when he did so without supervision Jake threatened him, David wasn't a member of the group he really was just the new guy nobody really liked (. . . but he didn't have to keep saying it!)
Honestly I do wish it had worked out with David, he had a certain charm before he went JtHM on everyone and you know Rahcel must have wanted to ask him how he got a friggin' Orca morph. But then agian I wish a lot of things, doesnt mean they're going to come true. In the end David done effed up, it wasn't entirely his fault that he did, but he did, and how could they trust him to stop?