The poet William Blake once famously said—or infamously, depending how you look at it—that in his portrayal of Satan, Milton was of the devil's party without knowing it. The Romantic generation, taking its cue from Blake, hailed Satan as a hero because they saw in him a proud, defiant individual with a strong personality, battling against the seemingly arbitrary forces of authority in the shape of the Judeo-Christian God. Romantics like Shelley, who developed a similar theme in Prometheus Bound, believed that Milton's Satan epitomized their constant struggle against the authoritarian structures of society which they felt held them back, stifling the creativity of the individual artist.
It is important to recognize that this portrayal of Satan in Paradise Lost is by no means universally accepted. Indeed, the general consensus among literary scholars is that Satan is more of a parody of a tragic hero rather than an actual one, not least because he isn't a fundamentally decent character brought low by hubris. In fact, as portrayed by Milton, he was never a decent character in the first place.
Whether or not Satan is a hero is open to debate. Milton had no intention of turning Satan into a hero.
However, there are several reasons Satan has been considered a hero. The Romantic poets embraced Satan as, like themselves, a supremely talented and flawed individual, crying out against convention and stifling religious orthodoxy. They, too, wanted to shake a fist at God and his tyrannies. They took to Satan's idea that it is "better to reign in hell than serve in heaven." It was better, they thought, to go down fighting than to remain servile. Today, too, there is a strand of admiration for the figure who dares to take on God himself.
Second, Milton's depiction of God—which should be differentiated from the God of the Bible, as the two are not the same—is an unpleasant creation. One tends to want to root for Satan against this vengeful divinity. God holds all the cards and is laughing up his sleeve as he lets Satan think he can rebel, knowing that He will trip him up even more badly than before, and kick him even harder when he is down. The inevitability of his doom and yet his zest and willingness to fight it can make Satan look like a tragic hero.
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.