Im sure how well it would mesh with the mechanics but The Vermin Lord (I think its called) would be cool if you want to 'get your plague on'.

The problem with Druids really is that an evil druid is still a druid, things like Blighters and Ur-priests happen when they turn against nature or religion but thats an anti druid rather than an evil druid.

I would personally reccomend sticking to whatever druid build you enjoy but selecting your spells and wildshape form to be more gribbly and evil