That's at end game, though. If the mafia gets to the point of the 'auto-win' and the DA's still alive, the mafia still has a problem ahead.
3 townies, 2 mafia+ DA Game hasn't ended.
Night: Mafia boss kills mafia member when trying to get the DA
Day: 3 townies, 1 mafia + DA
DA SCREAMS: "Kill (Mafia boss!)" Since there won't be a night the DA can do this safely (so long as they don't say who they are they don't break the 'secret' rule)
Town wins
Thus the mafia can't simply just kill all of the mafiosos. They need to umm... anti-scumhunt to figure out who the fake is. If they do, they can wipe out the DA either at night or in the day via a 'bus'. Probably better in the night to avoid the DA talking too much.
OO, cute situation:
Mafia member: "Man, they are getting too close to us. We need to buss someone. hey man, let's bus you out. That alright?
DA: Umm... you sure we need to buss?
Mafia member 2: "What's wrong? Nothing wrong with dying....RIGHT?"
Sidenote: when sending the mafia PM, just mention who your buddies are.. don't mention WHAT they are. Thus you aren't 100% sure the mafia boss is who he is. THUS.
"Mafia Boss: Clearly the boss is not a double agent. You are no longer useful. I assign myself the nightkill and kill [my mafiosos]."
Mafia member 1: WAIT! How do we KNOW you're the boss. You could be the agent!
DA: YA! Kill the agent!
IMO if one mafia-agent exists, there should be an agent in the other mafia as well. Thus you get two agents in both chats. If one speaks up, the other will do the same, resulting in both agents dying. Mutually Assured Destruction, along with PMs not available, will keep both in line.
Again, remember that the DA has a Survive requirement. They lose if they get caught and die, even if their side wins for it.