In broadest of senses, Milton's purpose in Paradise Lost is to “justify the ways of God to men.” He seeks to explain why Satan is seen as the cause of evil within human beings. Exploring and illuminating the "infernal serpent" becomes part of Milton's purpose in his characterization of Satan. Doing so reveals the pride and haughtiness that explains both the fall of the archangel and the human beings who succumb to him.
Such elements can be seen in lines 120- 160 in Book I. When Satan and Beelzebub speak to one another, it reveals how Satan sincerely believes "all is not lost." His characterization is shown as one that rejects contrition and humility. Rather, there is a staunch defiance within him, one that stresses that "to be weak is to be miserable." Satan's envious and coveting characterization is shown in his articulation of the divine as one "Who now triumphs, and in th' excess of joy/ Sole reigning holds the Tyranny of Heav'n." It is Satan's pride that enables him to compare God with a tyrant.
While it is evident he is "in pain," Milton does not miss the opportunity to show him as resolute in his "infernal" spirit and establish his purpose as antagonistic to redemption and submission. In contrast to this, Beelzebub's characterization is one that is willing to contemplate the possibility of another path. Beelzebub's purpose is to provide the potential counterpoint to Satan, to suggest that there might be a way out of the condition in which both angels find themselves:
What can it then avail though yet we feel
Strength undiminisht, or eternal being
To undergo eternal punishment?
The asking of purpose and relevance in why they do what they do is significant. Beelzebub's characterization is less forceful and confident than his leader. It depicts that even those in mired in evil and transgression can seek another path. It suggests that the glory of the divine can reveal itself in any situation.
Beelzebub's purpose in offering another alternative frame of reference helps to substantiate Satan's defiance in the face of the divine. Satan reaffirms his commitment against the light of the divine: "...but of this be sure,/ To do ought good never will be our task,/ But ever to do ill our sole delight." The characterization of Satan is shown to be one committed to his own end. Satan's resistance and rebellion are shown in lies 120- 160. It is a condition that will forever prevent him from understanding the ways of God and prevent him from finding any peace. The chains to which he is physically bound are reflected in his own mental condition of defiance and antagonism. Lines 120- 160 in Book I help to bring out this purpose within his characterization.