It takes a little bit of effort to be a particularly evil (or good) druid in my worlds, largely because having an alignment typically requires actions taken specifically to have good or evil outcomes - and since nature does not make that distinction, but the character is making that distinction, we have a case where the character's goals are not in alignment with their patron power's goals, and that's a recipe of having your powers taken away.
In this case, do you imagine that "nature" is personified as a patron? So it can strip a druid of her powers if she strays from the path? Or is it more mystical, where a druid who is out of balance is unable to channel the power of nature due to their own imbalanced chi (or whatever)?
"Look, you were assigned to protect this forest from harm, excesses and abuse. You chose to implement this by systematically destroying every village within a day's ride of the border. That's... excessive, and not needed for your charge. We asked you to keep them from clear-cutting the trees, and instead you clear-cut them? That's not what we are about here in the Druid's Conclave..."
This implies that the druid hierarchy can strip members of their powers, like being excommunicated.
It can be managed if you have druids representing specific aspects of nature, rather than *all* of nature at once. A druid focused on storms can rip the crap out of human towns, so long as they are being all stormy about it. If they do it by poison... well, again, they are going out of their bailiwick.
This is an interesting idea, similar to what [MENTION=1125]Tonguez[/MENTION] added a few posts further on:
A Shark Druid might be a fierce and blood-frenzied eater of humanoids, a Rot Druid might have a mission of encouraging disease and decay, they would be antagonistic to humanoids but in their own minds they arent evil.
I'm intrigued by the idea that any given druid might specialize in a particular aspect of nature. That could create fun options in the game world.
As a random aside, I've been watching Avatar: The Last Airbender with my kids and, serendipitously, we just saw "The Swamp" (S2:E4) which features a water bending tribe in a swamp that had some druidic aspects.