Two scenarios:
1a) we leave him alive and mislynch. In this case, we lose, assuming two scum.
1b) we leave him alive and lynch correctly. We go to tomorrow in the same situation - 1 survivor, 1 scum, 3 townies
2a) we lynch him. He's scum and we're smiling.
2b) we lynch him. He's a survivor and we go to tomorrow in an equivalent situation to 1b
All this is assuming two scum. If we have one scum, we get one mislynch then we're in the same situation. If we have three scum, then we've already lost - it's just a case of scum logging on and putting someone at l-1, and ua playing to his wincon hammers for a scum/survivor victory. So we can discount that.
UA is either survivor or scum, and both are equivalent for endgame purposes.