The Romantics were interested in Milton's Satan because he resembled a liberal and a rebel, but he remained for them a diabolical figure. In his Preface to Prometheus Unbound Shelley neatly characterizes Milton's Satan, and argues that Prometheus is a more fit exponent for the treatment of the liberal political vision Shelley is seeking to express. I believe Milton would agree. Miton's Satan is an unsuccessful rebel, and Milton shows us why. And of course Milton's hero is the more morally intelligent and more politically capable Son.
From his preface to Prometheus Unbound, Shelley on Satan:
The only imaginary being, resembling in any degree Prometheus, is Satan; and Prometheus is, in my judgment, a more poetical character than Satan, because, in addition to courage, and majesty, and firm and patient opposition to omnipotent force, he is susceptible of being described as exempt from the taints of ambition, envy, revenge, and a desire for personal aggrandizement, which, in the hero of Paradise Lost, interfere with the interest. The character of Satan engenders in the mind a pernicious casuistry which leads us to weigh his faults with his wrongs, and to excuse the former because the latter exceed all measure. In the minds of those who consider that magnificent fiction with a religious feeling it engenders something worse. But Prometheus is, as it were, the type of the highest perfection of moral and intellectual nature impelled by the purest and the truest motives to the best and noblest ends.A little lower, Shelley has this to say on Milton:
We owe the great writers of the golden age of our literature to that fervid awakening of the public mind which shook to dust the oldest and most oppressive form of the Christian religion. We owe Milton to the progress and development of the same spirit: the sacred Milton was, let it ever be remembered, a republican and a bold inquirer into morals and religion. The great writers of our own age are, we have reason to suppose, the companions and forerunners of some unimagined change in our social condition or the opinions which cement it. The cloud of mind is discharging its collected lightning, and the equilibrium between institutions and opinions is now restoring or is about to be restored.