ATM it stands at:
3pts for killing Mastermind
2pts for killing infected
1pt for killing a survivor
1pt for lasting longer than a fellow survivor
Basically the key is to survive as long as possible and try and get some FFA-style frags of infected (fire the finishing shot rather than waste ammo/risk yourself hurting fresh-spawns). Personally I dislike this scoring method since I prefer playing as infected - playing a suicidal game (try to pummel mastermind each round) is almost impossible to finish outside of bottom place.
I'm still reeling from actually winning a game of RR. Infected is great fun, but what does it take to kill the bot..? He seems damn near indestructible. *Frowns*