why is Oliver kidnapped..?

Nancy tells Miss Maylie that she is the one who kidnapped Oliver and took him back to Fagin when he was living with Mr. Brownlow. Nancy explains that in the first conversation she overheard between Monks and Fagin, she learned that Monks had seen Oliver with the Dodger and Bates, and had told Fagin that he would pay him if he got him back and made him into a thief.