One possibility is that the druid doesn't have a specifically 'druidic' motive for being evil. Maybe they just like power, or think "If I was in charge, things would be better" and fight to destabilize an existing kingdom or take over some area. Sure, when they do take it over they'll treat the natural lands better than other overlords would, but that's not what drives them. Maybe that orc druid is using his abilities to help his horde succeed. Yes, when they raze all of the human settlements in the border lands it will also allow nature to flourish, but again that's not what's driving him. Or Sir LightyPantsTheBold wronged him years ago, and he's working to bring down Sir LightyPants and his allies for that. The slight might have involved hurting an area the druid was protecting, but again that's not the important part.