In Pride and Prejudice, why/how did Wickham try to take revenge on Darcy and why did it become impossible for Wickham to elope with Georgiana?
to take revenge for not allowing him to take up the living.
Mr. Darcy explains that after both his and Mr. Wickham's fathers pass away, it is left to Mr. Darcy to uphold the late Mr. Darcy's will and promote Mr. Wickham in his profession; ie: helping Wickham to find a location where he could take up his living in the religious order. However, Wickham comes to Darcy and explains that he has no plans to be a clergyman, but instead plans to study the law. Wickham asks instead for the money that would otherwise have been granted to him - Darcy gives him three thousand pounds. Wickham quickly goes through this money and petitions Darcy for more money. When Darcy refuses, Wickham travels to find Georgiana, it seems with the express purpose of convincing the young girl to marry him, and therefore use her inheritance to finance his exploits.
Wickham does not elope with Georgiana because Darcy visits her just days before they are intending to elope and she reveals the whole situation to him. Darcy, of course, stops the marriage from taking place. Darcy has always thought ill of Wickham because of Wickham's behaviour and conduct, and based on this explanation this seems to be an apt judgement.