Abel Magwitch is Pip's convict friend who vows to turn Pip into a respectable gentleman. Magwitch is yet another orphan who has absolutely no family member to befriend him. In fact, he knows nothing of his history at all (except for the simple concept of his name). Magwitch is always in trouble, even to the point where he is banished to Australia to herd sheep. When Pip and Magwitch meet again, the reader finds that Magwitch is Pip's mysterious connection to many things. There is a threat, however, that Magwitch will be captured again and sentenced to die; therefore, Magwitch uses the pseudonym of "Provis." Even though Magwitch is revealed and sentenced to death, he feels fulfilled because he has succeeded in his goal: transforming Pip into a respectable person.
In regards to his name, that is definitely an opinion question, but my thoughts are that Abel Magwitch was named so in order to stress his dual nature. He is both a "witch" and "able." He has elements of bad and good within him. He also has more connections than any other character. It also makes me think of the victim "Abel" in the Bible (where Abel is killed by his brother, Cain). Magwitch, in fact, is a victim of his unfortunate circumstances that he uses to promote goodness as far as he is able.
The convict whom Pip encounters on the marshes in the first chapter, Abel Magwitch has been the victim of the "prison of society" that Dickens frequently castigates in his novels. Born into poverty, in Chapter 40 of "Great Expectations" Magwitch answers Pip's question of "What were you brought up to be" by saying, "A warmit, dear boy." Later, as he eats with his knife, he tells Pip,
I'm a heavy grubber...if it had been in my constitution to be a lighter grubber, I might 'a got into lighter trouble.
As a child of the streets of London, Magwitch became a "varmit," feeding off whatever he could find, stealing from people, like a maggot--a fly larvae living on decaying matter. Used by Compeyson in his dastardly plot to rob Miss Havisham of her wealth and joy, Abel is the victim of evil, much like his Biblical namesake.
That Magwitch wants to be known as Provis indicates his desire to change his misfortune, caused by the "witch" of Fate, to the good as he provides for Pip's better existence is evident. On the other hand, he himself is a type of witch (a magic witch), an instrument of Fate, effecting the change of Pip's fortunes, both in positive ways--the money--and negatively--the disappointment of a convict as he benefactor rather than one of the upperclass.