Your short story sounds intriguing and interesting and you have the basic outline ready. The next stage is to develop the action and add some depth. Firstly try to remember what was in your head when you first dreamed this guy up as a character, and think about how his personality will influence the ending of the story. If he is repentant, then you will want to show that change taking place slowly.
Think about what kinds of experiences change hearts and minds and what programs are in place for criminals to make recompense to their victims. it is easy to walk away from a violent act, but definitely not so easy to have to come back and see what your actions have done and how that person is living. Perhaps you could write about some shock or surprise he had in this regard and how he went home a changed man, upset or distressed or even angry with himself. How would he show that? Maybe he would have a compulsive need to keep going back to see how she's getting on, and perhaps want to send her a signal even if it's anonymous at first? Then perhaps he gets up the courage to write, phone or speak, or leave a mysterious gift, a flower or special object.