In Great Expectations, why does Pip refuse to accept Magwitch's money?

At first Pip does accept the money, believing that Miss Havisham is the anonymous benefactor. However, once Magwitch appears on his doorstep and he reallizes where the money has come from, he feels that he wants no part of it. However, by the end of the novel, as he pieces all of the story together, he realizes that Magwitch is a much better person than he could have imagined.