Exactly, the picture is skewed...most of the Yeerks we met in the series appeared power hungry or ruthless, but they can't all be that way. Even those that were, like Eslin who wanted to kill Visser Three (there's rebellion right there) was doing so because he lost someone he cared for. There was that Yeerk scientist who was working on removing free will from humans, and he knew it was wrong and furthermore impossible, but he knew he would be killed if he failed or even refused. He asked the Animorphs if he could come with them, I believe...now that would've made an interesting storyline, whether he was made a nothlit later on or simply forced to starve...but the Animorphs just said no. Aw, well 
I can't picture the Ani's forcing him to starve, not at that point if he'd come with them and trusted them. However, I'm not sure if the nothlit thing would have occured to them either, and it is one thing to entrust Aftran with the morphing technology when she's proven herself, but that Yeerk? I imagine they may have given him a quick death, or perhaps passed him over to the Chee, who can generate Kandrona, as we see in number 10 with Erek. Either would probably be preferable to what V3 might do to him.
As Ax was narrating that book, it would also have been hilarious to see his reaction to having a Yeerk come with them!!
But yeah, we only see a handful of Yeerks in the series, and very few of those in any detail. I imagine Yeerks are as varied and complex as humans. To decide all humans were evil based on the actions of, say, Saddam Hussein and Hitler, would be erroneous, and IMO it's the same with Yeerks.
