I think some Yeerks became proud and greedy because a) they were influenced heavily by their society's propaganda or b) they were just crazy and power hungry. We do know that there were Yeerks who didn't like infesting people, and I think exposure to so many different host species (especially humans) helped the less proud and greedy psycho-Yeerks figure that they felt sympathetic to the host species.
Those who were aggressive probably weren't exactly helped by the strict, aggressive nature of Yeerk society and their biological lot in life, but since they weren't aggressive--and in fact were pretty peaceful supposedly, before becoming an Empire--across the board it's probably more a nurture (the government), not a nature (their biology), thing.