"I'm mad as Hell, and I'm not going to take it anymore!!"

How does Milton play with the original story in his epic Paradise Lost?  Who should we regard as his central character, protagonist, antagonist?  Is Satan a sympathetic villain, a tragic hero, a simple stock character, or a mere catalyst of action?  Given his liberal tweaking of the source material, what new questions and themes emerge in this version of Man's beginnings and Fall from Grace?