I think it's important to think of yeerks as being just like humans, equally capable of good and evil. During WWII alot of german citizens went along w/ their country's actions out of nationalism, being brainwashed, fear, etc. I think that eventually if all yeerks had suitable hosts the majority of the yeerks would have been fine living more peacefully.
If you look at book 41 as a vision of the future, it seems that the yeerks settle into a pretty peaceful existence after conquering the humans and andalites. I remember seeing a billboard for some sort of yeerk vacation? I forgot what that was. Also, they adopt human business ideals such as teamwork and communication.
