What made Milton able to portray a Satan that is so universal? Why do some feel he is the most accessible character in the poem? After reading about Milton's life, I think that it is because Milton was more sympathetic with Satan than with any other character.
Milton was fueled by ambition. He was determined to write the great English epic.He felt divinely appointed to be one of literature's greatest figures, and struggled with authority figures that tried to shape him differently. Magnify these characteristics-- the ambition, the eloquence, and the resistance to authority-- and we have Satan.
I'm not making the claim that Milton was a Satanist. He obviously was a Christian who intended to perpetuate Christ's teachings through this work and other poetry. But I think that there is an interesting relationship between Milton and his Satan, and that Milton likely saw in himself (and humanity as a whole) the characteristics that caused Satan's downfall. This made him able to so convincingly portray a figure who constitutes a massive part of our theology.
As the debate continues, maybe the truth is that Milton struggled just as much as the rest of us to define who the real tragic hero is in Paradise Lost.