Milton's Satan, like Blake's is the only full fledged personality in Hell. The other demons are more like aspects of his potency than fully realized characters in and of themselves. When Satan finds his mill workers celebrating with drunken revelries, he cries and comes back to Los, convinced that Palamabron is behaving badly.
Satan is not the god of desire, he is the god of moral imposition and austere self-hood. He wants everything to run according to a specific order. He is a tyrant, a megalomaniac. But as Blake reminds us, one man's Satan is another man's God.
M9.19; E103| For Satan flaming with Rintrahs fury hidden beneath his own mildness
M9.20; E103| Accus'd Palamabron before the Assembly of ingratitude! of malice:
M9.21; E103| He created Seven deadly Sins drawing out his infernal scroll,
M9.22; E103| Of Moral laws and cruel punishments upon the clouds of Jehovah
M9.23; E103| To pervert the Divine voice in its entrance to the earth
M9.24; E103| With thunder of war & trumpets sound, with armies of disease
M9.25; E103| Punishments & deaths musterd & number'd; Saying I am God alone
M9.26; E103| There is no other! let all obey my principles of moral individuality
M9.27; E103| I have brought them from the uppermost innermost recesses
M9.28; E103| Of my Eternal Mind, transgressors I will rend off for ever,
M9.29; E103| As now I rend this accursed Family from my covering.
Satan here tries to supplement the creator, he is a false God, mimicking the God of the Old Testament; he passes these laws down out of jealousy for Palamabron's "ingratitude." Satan alone is God, and this individuation which exists at the expense of all others, will prefigure the wars that are fought. It is this desire to put all people under one name, the insanity of unity, the insanity of unifying all human consciousness, which Paul himself is guilty of, that drives men to destroy men.
Satan's bosom grows opaque so that his heart, and motivations may not be questioned. He offers everlasting death to those who disobey him and thus the world of Ulro is born, out of moral slavery, and Satan's jealousy.
Satan's role now is to obscure vision, his motivations are megalomania and enslavement. What is lost is the ability to see through language. Instead language acts as a barrier to the external world, a barrier which must be broken in order to reclaim the divine vision, which is the sole providence of no single religion, but rather all religious systems.
It is then that Blake mentions the Covering Cherub which the God of the Old Testament placed before the Tree of Life after the fall. This re-echoes the fall from Eden, and Urizen's fall from Eternity, which is explicitly spelled out by Blake: Satan is Urizen.
So Satan's role is to divide the whole of humanity into pieces, and then set the principle parts at war with each other.