I'd check the Scarred Lands for some inspiration as well. I think for the social Darwinist sort of druid, I demonstrate it through an acceptance and callous disregard for those weaker who suffer. A village that burns the sick, old and infirm at the druid's command, in order that strong may prosper with the blessing of whatever force or nature the druid serves. I'd have scenes where the character finds pits of the bones of infants and young children who were deemed unworthy to grow because of "imperfections" or weaknesses. I'd also have him fawn over the strong, and the "perfect" or "beautiful". I'd shower the PCs with compliments if they have high attributes or continuously over come obstacles to confront the druid. If they defeat him, and should he lay dying because of them, I'd have him show immense respect to them, since they proved the stronger, and have come to understand "his lesson."