The encounter between Pip and the runaway convict at the beginning of the novel and his promise to him is crucial to the plot of Great...

Pip promises everything the convict demands because he is terrified. This is what the convict demands.

“You bring me, to-morrow morning early, that file and them wittles. You bring the lot to me, at that old Battery over yonder. You do it, and you never dare to say a word or dare to make a sign concerning your having seen such a person as me, or any person sumever, and you shall be let to live. You fail, or you go from my words in any partickler, no matter how small it is, and your heart and your liver shall be tore out, roasted, and ate."                                                                                               Chapter I

In Chapter III Pip brings the convict a generous amount of food including a whole pork pie, along with brandy and a file from Joe's workshop. The convict is eventually caught and taken back to the prison ship. He disappears from Pip's life. Some years later Pip is informed by Mr. Jaggers that he has "great expectations." A benefactor, whose identity is to remain a secret, wants Pip to be trained to become a London gentleman. Pip naturally assumes that the benefactor is Miss Havisham, but he cannot question her or Mr. Jaggers, who becomes his guardian.

Then in the critical and marvelously written Chapter XXXIX, Pip, who is now a cultured gentleman, is amazed and horrified to learn that his real benefactor has been the convict he helped in Chapter III when he was a little boy. The convict, whose name is Abel Magwitch, had been transported to Australia and has come back at the risk of being hanged to see the boy he made into a gentleman.

“Yes, Pip, dear boy, I've made a gentleman on you! It's me wot has done it! I swore that time, sure as ever I earned a guinea, that guinea should go to you. I swore arterwards, sure as ever I spec'lated and got rich, you should get rich. I lived rough, that you should live smooth; I worked hard that you should be above work. What odds, dear boy? Do I tell it fur you to feel a obligation? Not a bit. I tell it, fur you to know as that there hunted dunghill dog wot you kep life in, got his head so high that he could make a gentleman—and, Pip, you're him!”

The message of Dickens' novel is that the fine ladies and gentlemen all owe their superior refinement, their luxuries and their privileges to the ignorant and exploited lower classes represented by Abel Magwitch. Pip realizes that he has become a useless fop incapable of doing anything but act the part of a gentleman. He had always assumed that Miss Havisham was paying for his education and culture in order to make his a suitable husband for Estella. This dream comes crashing down, along with the realization that he is virtually the property of a well-meaning but hopelessly ignorant and uncouth convict who has been a criminal since boyhood and who has returned to England in blatant violation of the law. Pip is now obliged to harbor Magwitch and to have him as a constant companion for the foreseeable future. As Magwitch tells him:

Look at your clothes; better ain't to be got! And your books too,” turning his eyes round the room, “mounting up, on their shelves, by hundreds! And you read 'em; don't you? I see you'd been a reading of 'em when I come in. Ha, ha, ha! You shall read 'em to me, dear boy! And if they're in foreign languages wot I don't understand, I shall be just as proud as if I did.”

So Pip's encounter with the escaped convict on the marshes in the opening chapter is to affect his entire life. He becomes Magwitch's idea of a gentleman, which is like a spoiled and haughty parasite. He cannot even feel that he deserves all the money that Magwitch has spent on his "education" because Magwitch completely misunderstood Pip's motives in helping him as he did. 

“You acted nobly, my boy,” said he. “Noble Pip! And I have never forgot it!”

Pip was not acting out of compassion and generosity. He was only doing what he had been forced to promise because he was frightened out of his wits. Magwitch had a better opinion of the little boy than he deserved, just as he has a better opinion of the gentleman he has created with all his hard work and privations than that gentleman deserves now. Pip is thoroughly ashamed of himself, and Dickens intended at least some of his readers to share that shame. If some people have too much, it can only be because others have too little.