One of the most significant events in Oliver Twist is the death of Nancy.
At first, Nancy seems like a hopeless criminal. She is a thief and heavily hinted to be a prostitute. She helps Fagin and her lover Skyes kidnap Oliver when it seems he will be able to escape their negative influence. However, Nancy sees in Oliver the innocent child she once was before she was corrupted and eventually decides she wants to help him live a better life, even though this will endanger her life. She secretly tells Rose Maylie and Mr. Brownlow about what happened with Oliver, but Fagin finds out and then has Skyes murder her for her treachery.
Her death is significant to the narrative, both in terms of plotting and theme. Nancy's murder is what sets the mob on his trail and leads to his death. It also helps ensure that Oliver will be saved. It also concludes her character arc: she goes from being a cynical, unrepentant criminal to a heroic, Christ figure redeemed by love.
Thematically, Nancy's death relates to the book's ideas about the connection between class and character. Though living among the lowest rungs of society as a prostitute, Nancy still has a conscience and is capable of Christian sacrifice—something many doubted was possible for criminals during the Victorian period. It was believed that some people of the lower class were naturally inclined to criminality (see the constant statements of Oliver being "made for the rope" by the workhouse authorities).
Nancy is one of the bravest, noblest characters in the novel, and by having her risk her life to save another person, Dickens is making a statement about human nature and class.