As with most dilemmas, the correct response for the Townie(s) is to take a third option and No Lynch. There is absolutely nothing the scum can do in this situation but follow suit.
At Night, the dilemma shifts to the scum. The only way for either scum to win is for them to shoot the other scum. But if both scum do this, then the Townie(s) will be the only player(s) alive, and the Town will win. This is a disheartening outcome so late in the game!
The other option, shooting the Townie, is clearly suboptimal. Going with the three-player example above, if the Mafioso shoots the Townie and the Serial Killer shoots the Mafioso, the Serial Killer wins (and the Mafioso feels dumb afterward). Thus, to the scum, the choice is between giving victory to the other scum or the Townie.
It is worth noting that depending on the moderator, if BOTH scum shoot the Townie, the result is a draw. Since a draw is less than a win, there is no real reason to not shoot the other scum. (Or is there? See Caveats below.)
Thus, the Town will always win.
But now the situation seems hopeless for scum...