Jake: Flushing the 17,000 yeerks. Not even a question.
Rachel: Surprisingly enough, I can't think of anything that really stands out. A murder here, a few lives taken there, and it adds up to be a lot, but there's no one thing that she's guilty of. Threatening David was a low point, sure, but she's done worse overall. If I had to pick one thing, though, I'd probably say killing Tom.
Tobias: Nobody else remembered that he made the call to wipe out an entire race? Anybody remember the Mercora? True, he had no choice, because it had 'already happened,' but he still made the call, and that was pretty cold.
Cassie: Giving Tom the morphing cube. Indirect as it may have been, I consider her, and not Jake, responsible for Tom and Rachel's deaths, since neither of them would have needed to die if Jake had been able to stop Tom (Jake wouldn't even have needed to kill him to get the cube back, IMHO). After that, Jake was just making the only decision available to him. And without Rachel dying, Jake wouldn't have been desperate enough to think of flushing the 17,000 yeerks, so Cassie shares some of the responsibility for those lives, too.
Marco: Shoving his own mother off a cliff.
Ax: Even considering allowing the Andalites to 'quarantine' earth in book 52. The fact that he considered dropping a nuke on one city doesn't even compare to the fact that he was actually thinking about betraying our entire freaking planet. To be honest, though, I really thought that book was out of character for him. He'd decided as early as book 18 that his loyalty lay with his human prince rather than any Andalite commander.
