It's the same reason there's this crazy Star Wars fan theory making the rounds across the net, that Jar Jar Binks was actually a Sith lord, and all the weird little accidents and coincidences that seem to just happen to work out in his favor were actually his doing all along. People don't think that because it's the explanation that makes the most sense; it isn't. People latch on to this idea because it fixes something that was bad.
And now Cassie will forever be the Jar Jar Binks of Animorphs to me.
I'm not buying her being a psychopath, I think she's more the blind idealist/fundamentalist (not sure the right word for this). Basically, she has her worldview, you can't change it, she won't consider she can be wrong, or someone else can be right. If you follow her view, she's your best friend, if you dont, she hates you or maybe just really dislikes you, and you're the enemy to either be converted or fought. That's not really psychopathy, that's just idealism gone really bad.
The only one that ever really struck me as a psychopath was David. He was just off. Like, he didn't have morality, but knew he needed it to fit. So he substituted rules for morals. He can't kill people because it's against his rules, but if they're in morph, they aren't human, so there's no problem. That kind of stuff. That was downright eerie.
(I caught up on a lot in this thread)
Tobias having aspergers? Well, no. I'm not that up to speed on it, even though I recently found out my parents had me tested for it when I was really little. Twice. (Apparently I was shown pretty conclusively to not have it both times). But, I don't really think he ever shows any traits of it except not fitting in. Ax seems a decent fit for it. I never really considered it, since he's an alien, he shouldn't get all that human stuff... except other aliens in the series all seem to get the things Ax doesn't after being with humans for a little bit of time. Ax spends years with humans and never gets any closer to catching on. So, yeah, I'm starting to wonder of Ax does have autism.