Arguably, Fagin is presented as being just as villainous as Bill Sikes in this novel, whereas Nancy is clearly a character who has measure of goodness to her. Note how the three are presented in Chapter 16, which is when Nancy has just managed to "reclaim" Oliver from Mr. Brownlow, effectively abducting him back into the "care" of Fagin. When Oliver realises his fate, he tries to run away from the house where he is held. Nancy's reaction to this is to jump up and to try and stop Bill Sikes from letting his dog run after Oliver with the following words:
"Keep back the dog, Bill!" cried Nancy, springing before the door, and closing it, as the Jew and his two pupils darted out in pursuit. "Keep back the dog; he'll tear the boy to pieces."
It is clear that she feels very attached to Oliver and will not stand for him to be hurt as long as she can do something about it, even if this means that she herself is physically abused by Sikes, as he does when he "flings" her across the room. Sikes on the other hand says that it would "serve [Oliver] right" if he was savaged, and Fagin, when he has regained Oliver, is quick to punish him physically. The text therefore presents Fagin and Sikes as being of a similar level of villainy, whereas Nancy is clearly presented as being a character with some measure of good in her, as her subsequent actions in the novel demonstrate.