In William Shakespeare's Sonnet 144, the author's intent seems to be describing two sides of his nature: that which is good and that which is dark, a common topic.
One is reminded of Marlowe's Faustus (in his play Doctor Faustus) as he struggles between what he knows to be good (represented by an angel hovering over his head) and the temptations of evil (represented by Mephistopheles, the agent of the devil).
In Shakespeare's poem, the same struggle is the topic of the fourteen lines of his sonnet, and the two sides involved in the fight are introduced in the first line:
Two loves I have of comfort and despair...
This line notes that the speaker is undergoing an internal struggle. The good is represented by "comfort," while the bad is presented as "despair," a lack of hope. The second line conveys the movement back and forth between these two extremes that "urge" him on. Line three notes that that which is good is pleasant to look at ("fair")—maybe the spiritual aspect of a person, while the other side, the "worser spirit" is a woman, "colour'd ill" (perhaps dark-haired and sultry) maybe the physical. This may well allude to the man being in control—and his appearance an indication of his virtuous spirit—while the antagonist is presented as a woman, often seen as a temptress, a seductress—one that leads a man to his physical downfall—unable to resist the lure of the flesh. This comparison might well be an allusion to Eve in the Garden of Eden in the Bible's book of Genesis. Light is often associated with good, while the dark is associated with evil.
As the struggle continues, the speaker notes that the female within (the darker spirit) tries to "corrupt my saint to be a devil." This would bring to mind perhaps the illusion of a witch believed during Elizabethan times to be female in nature and dedicated to winning one (often seen in Shakespeare's plays as a man) to his eternal destruction. This is how Shakespeare introduces the struggle within the man, in the first two quatrains (eight lines in total) of the sonnet.
In the third quatrain, there is a shift in the author's attention. The speaker notes in lines nine and ten that he cannot be sure whether the goodness in his soul will be corrupted:
And whether that my angel be turn'd fiend
Suspect I may, but not directly tell...
But he notes that both the two opposite powers represent opposite sides of himself. They are "both to each friend," meaning that they coexisit, and perhaps more than that, get along well. However, in that each represents something far different, the goodness inside him finds the darkness a version of hell, as the darkness would find the goodness her form of hell.
The rhyming couplet at the end summarizes the speaker's thoughts: until one side wins, the other side will exist in a state of doubt...at least until the "bad angel fire my good one out." While the speaker notes that he is doubtful as to the result of the battle, he seems then to contradict himself in the last line, expecting that the matter will be settled when the "bad angel" overcomes the other. He never presents the idea that the struggle could end when his "good" angel defeats the dark one. Perhaps in this way, the speaker recognizes the nature of man, noting that too often, regardless of one's intentions, the darker side often does out. We might assume that the speaker expects to fall victim to the dark...perhaps even wants to.