Well, what if you were in his place?
Look, without any warning, you suddenly get between a battle with a handful of Hork-Bajir, two Andalites, a Tiger, a snake, and a Grizzly bear. And suddenly. your humanity is taken away by 4 kids, a blue deer, and a hawk by explaining to him that the Yeerks are here and his parents are probably infested, and they force you to fight against these alien slugs.
Wouldn't you be shocked? Plus, the guy was already messed up from the start. Kids his age treat him the way bullies treat Tobias. Even worse. Unlike Bird-boy, the teachers don't like him and girls don't feel sorry for him. He doesn't had have any friends either since he was always moving. Add the Yeerk stuff and the guy would go insane. Jake, Cassie, and Rachel all had good environments to start with.
Sure, Marco and Tobias have been messed up too, but thanks to the other Animorphs, they have eventually coped up really well.
The weasel... ermm.. guy deserves a chance. It would also be great if, like Truth said earlier, if he had stayed longer and betrayed the Animorphs later in the series...