In some states if two or more people attack an individual it is legally considered a lynching. This distinction can be to the victims advantage. In a one on one fight the attacker is killed the survivor has to prove that they believed their life was in danger at the time. In the case of a lynching you don't have nearly the same burden of proof.
This is an especially good thing if you are a trained martial artist (even a beginner) because judges will generally assume that you had the advantage in a one on one fight even if this wasn't really the case. It also means that you don't have to hold yourself back to nearly the same degree during the fight if it is a lynching. Leaving them crippled, comatose, or dead need not send you to jail (assume you were acting in self-defense and they posed a real threat). The hardest thing you will have to deal with afterwards is your conscience, which shouldn't be too bad since they were trying to pummel, maim, or possibly kill you, to begin with.
Check with the police or a lawyer before assuming this law is in effect in your state.