Or HaChaim on Genesis 19:17/אור החיים על בראשית י״ט:י״ז

1 אל תביט אחריך, "do not look behind you!" The reason for this is simple. We believe that by looking at a person's face one (a צדיק) can detect whether he is good or evil. If Lot were to turn his face towards the city his sins would be reflected on his face and there would be no way he could have been spared.