Since this is a pretty important mechanic that I haven't seen implemented in other games, I'll explain how it works.
Each faction, Faction 1, Faction 2, and town have a factional bulletproof. This means that the first time that one of their members is killed, then that factional bulletproof will be used. However, if a protective role is being used instead, it will take priority, meaning that the factional bulletproof will not be used.
For example:
On N1, Faction 1 shoots town. No one dies, but the factional bulletproof is used. On N2, Faction 2 shoots town. The townie dies.
OR:
On N1, Faction 1 shoots town that is being protected by another townie. No one dies, and the factional bulletproof is not used. On N2, Faction 2 shoots town, and the factional bulletproof is finally used.
I know that this seems quite complicated, but it was the only way I could figure out to balance a game so that scum couldn't achieve parity too quickly and crosskills wouldn't swing the game so dramatically, while also making it so protective roles were still useful.