Give it the twist that Mafia knows who are the traitor.
Included immune at night, however not infiinte - town dies, -1 HP.

If he has, let's say default 3 with option 4 - if enough town dies, he'll commit suicide.
To balance it, leavers N1/2 are counted as non-town deaths.
Max -1HP per night.

That way, if all town dies by a single Arsonist - it's only 1HP, so you'll still have the chance to revenge on the mafia.