Mr. Wickham's primary motivation in the novel is to marry for financial security. He is arguably the most selfish character in the novel. When he is first introduced he is presented as a charming man who has definite romantic interest in Elizabeth, but once it becomes clear that she will not have much of an inheritance, he quickly shifts his attentions to Miss King who has a comfortable amount of inheritance in the bank. Lizzy isn't all that interested in Wickham and actually defends this action to her aunt when they talk about what Wickham has done. Lizzy fully understands the necessity of financial security, even if she isn't willing to marry for it. At this point in the novel she still believes Wickham's story that Darcy hasn't been fair in regards to his inheritance from the Darcy estate.
As the novel progresses we learn that the relationship with Miss King has fallen apart and that Lydia is very interested in Wickham. Unfortunately, Lydia is very naive and foolish -- she runs off with Wickham and creates a complete scandal. Wickham has no intention of marrying a Bennet daughter -- they have no money! Wickham only agrees to marry Lydia when Darcy bribes him with an incredible amount of cash and a commission in the military (higher rank). By this time, Elizabeth knows that whole story of Wickham's lies about Darcy, and she is appalled that her sister now has to be married to the man, but she is relieved that a family scandal has been avoided. When she later learns that the marriage came about because of Darcy, her feelings for Darcy are cemented. She ultimately marries him for love and tolerates Wickham as a brother-in-law.