There's something that bugs me everytime I touch this book
Captain tratior looked up to Elfangor "I'll believe he broke the rules, I'll never believe he did wrong" wasn't it?
So wtf? why would he sell out to the Yeerks
Did anyone catch a motive that I missed, I mean seriously, why captain traditor dude, why?!
To restate what I said in my video review of the book, the only thing we really know is that the captain served under Elfangor and was close friends with him, close enough to believe that Elfangor could do no wrong, even if he broke Andalite law. Now, as we know from the Andalite Chronicles, Elfangor was a deserter. Given the option of returning to the Andalites or going with Lauren to Earth, he chose to escape the war and disown his people. When he did rejoin the Andalites, he did so unwillingly, having been mislead by the Ellimist and leaving a wife and son behind.
It's therefor not unreasonable to assume that Elfangor retained some resentment over the whole situation, and some of that could have rubbed off on the captain here, given him the germ of the idea that maybe the Andalites aren't the right horse to back. There's more to it then that, I'm sure, but it's a start.