In Book I, having been cast out of Heaven because of his pride and envy, Satan takes the form of the serpent and tempts the "Mother of Mankind." But, his war in Heaven is a "vain attempt" and God defeats him, casting Satan into Hell where he encounters the other "Rebel Angels" such as Beelzebub, one who is his equal. In his "obdurate pride," Satan claims that he has not lost all, for his will is "unconquerable" and he will be revenged. For, he will never bow to God because

That were an ignominy and shame beneath
This downfall; since, by fate, the strength of Gods,
And this empyreal sybstance, cannot fail

To become subservient to the Heavens is a greater downfall than what has occurred, Satan contends.  Above all else, Satan is proud.  He believes that the mind "is its own place" and can make a "Heaven of Hell" or a "Hell of Heaven," so to reign somewhere is the important matter.  Thus, Satan concludes, it is "Better to reign in Hell than serve in Heaven."

After his defeats, Satan remains proud and unrepentant, choosing to be the head angel of darkness.  He is, indeed, unyielding in his insistence upon serving no one.  Even when Beelzebub tells him that God is omnipotent, Satan insists that they can still turn good deeds to evil and subvert God's goodness.