Hell nun was an evil professional punisher in Metropolis. She tried to kill Superman because of his supposed "immorality".
When Hell nun fought Superman, She took advantage at the first, led Superman into a trap, kill Superman's son and she nearly killed Superman himself with her kryptonic pistol. Her victory was near but Superman cannot be easily defeated. It was at this moment, when all hope had faded, that Superman showed the secret part of his body, succeed to desorientate the Hell nun and killed her by a single punch.