Well, I'd say it depends really on your idea of "dark".
Druids have a deep, ingrained sense of balance in the world. If you mean along the lines of an anti-hero and/or lone wolf (er, bear... cat... tree... owl... whatever...), its very possible. The druid could believe that the only true way to achieve balance is to force it. From what I've seen at least, druids are rather passive in their changes, only scaling it up when the situation becomes dire. But a darker druid could cut straight to the chase, slicing the bud right off.

If you mean a more evil druid, same thing basically. Take Malygos for example, his best idea to save the world, to him at least, involved tearing the gift of magic from mortal hands. A more evil druid could be bent on exterminating all sentient, greater life (I.E. the player races and others of equal intelligence), all just to restore perfect balance. The latter is probably considered cliche'd as it follows the "road to hell" saying, but sometimes it really works out well.

I don't really use the roleplaying realms on WoW though, and the last class I'd use for roleplay would be a druid so I wouldn't be too sure on all this. I'm more of a paladin person. =P