The details of how Angelus came to meet the girl are sketchy, whether he picked her up himself or she was a "Gift" from Darla is unconfirmed, but what we do know is that when Angelus met the young Romany (Gypsy) woman that night, his unlife was to change forever.
The young woman was from a Romany tribe called the Kalderash and was very popular. In fact, she was their favourite daughter and they were outraged when they discovered her brutal death at the hands of a vampire. As a means of punishing Angelus for his crime. They cursed him with a soul.
Suddenly Angelus was hit with over a hundred years of guilt at his bloody and brutal past. Angelus had been a killer, and he had done it not for food, or for survival... but for fun! A fact which shocked him when his soul was returned.
He turned to Darla for help, but she was disgusted with him. She tried to convinse him what he did was alright, that feeding on humans wasn't a bad thing, they deserved it. Trying to please her Angelus kept feeding from humans, but only from those whose deaths he felt he could justify. Murders, criminals, anyone he could tell himself was bad. Darla however caught on to him and demanded that he feed from a child. The guilt was too much though and Angelus refused. Darla banished him from her side. Told him never to return to her, unless it was without his soul. Cast aside by his family and shunned by humans, Angelus wandered the earth for over a century, trying to find peace.