[Shadowstar]

Or just have a seperate side for them...

If you're a merc, you join the mercs side and then have special rules that apply to you. If you're a backstabber, you get "exiled" to merc status when you do your dirty little act, and probably get saddled with a nice little bounty just to make your new lifestyle change a little more interesting?

I just hope the Firefly MMO doesn't allow players to be reavers. I say this despite the fact that most 13 year old's tend to play MMO's that way (dirty little griefers) and they'd probably play the role well, it's just that there's something wrong about a reaver calling you a "flaming f***-tard" and then proclaiming itself "teh 1337 mA5tAh"...