Whether Satan can be called as the “Hero” or protagonist of John Milton’s Paradise Lost, has always been a struggle between moral/religious text as well as true literary interpretation. Satan is undoubtedly evil, corrupting humankind. And so in this context, he should always remain anti-hero with demeaning qualities that are unworthy of redemption.
In this sense, even though other characters like Son of God, Adam, Angels, etc. are suited to get the heroic status, Paradise Lost is not, ultimately, about them. Paradise Lost is the story of Satan. He is the focus of the writer as well as the reader. Milton’s Satan is a powerful, compelling character with extreme courage and pride, who fights and risks everything for a cause he believes in, so much that he gains reader's sympathy, who tend to associate with his failure and pain.
In a way, he resembles a tragic hero as he has a tragic flaw, hubris. Even though he is not as powerful as God, he goes beyond his limits, starts a revolt against his tyranny and oppression, and even hopes to overcome him. He is a leader with a plan. And like an admirable leader, he consoles and inspires fellow fallen angels, and asks them to not accept defeat and continue the war. Milton carefully projects God as wrathful, unjust and punishing, something that might have pleased his English Protestant audience.
Literary geniuses like Shelley and Blake considered Satan as the hero of Milton’s Paradise Lost. But even if his heroic standards are debatable, at the very least, Satan can be seen in a very complex, vivid light with not just contemptuous, antagonistic qualities, but also fascinating heroic elements.