How is Satan a hero in Paradise Lost by John Milton?

Because he rebels against God, who is a tyrant. The romantics and French revolutionaries certainly saw it that way. He actually gives a speech to his minions, talking about how he'd rather spend his days in hell than be subservient to a tyrant. It's been a while since I read it, but that should probably ring a bell for you. That whole scene is full of evidence for that.