What can be used for good, can be used for evil.
Super Saiyan analogy - To become a Super Saiyan, one must have a pure heart (this can mean pure good....or pure evil)
Jedi analogy - Balance must be preserved. Tauren/Night Elf druids represent the balancing - restorative aspects of nature. Trolls could represent the destructive - chaotic aspects of nature (like naturally occurring wildfires, or when a squirrel gets pwnd by a hawk)
So....Trolls already have the ability to take on animal forms (shape shift), are vile and nasty hearted (purity), are already close to nature (shamans), and would introduce balance to the druid arts (nature's destructive/chaotic aspects). Sounds plausible to me.
Thoughts?