Nancy is a prostitute and destitute who is also a part of a gang of thieves led by a man named Fagin. In that gang is also her boyfriend, Bill (Sikes). This gang has a fixation on Oliver since he was once asked to pic-pocket for them, and escaped. Since he knows of their existance, he poses a problem. Most of the story consists on Oliver being chased by Nancy and Bill to be returned to Fagin.
The death of Nancy came as a result of what seems to be a change of heart on her part which is an ironic twist of her character. She knew that there was another plot to take Oliver away from the Maylies, a family which took care of him after yet another escape from the gang. Nancy informed the family about this plot, hence betraying the gang. Sikes murdered her when he found her.
Nancy is a complex character because she is a mix of both good and bad. She is obviously a criminal, but there is a depth to her character after what she did to save Oliver. One wonders if she is actually a victim of circumstances rather than a cold-blooded thief. She is a representation of what happens when people simply do not have a chance to grow and develop in a different environment. The classical case of nature versus nurture.
Nancy was with fagin from small days.Bill Sykes killed her because she tried to save Oliver.