One theme of "The Rime of the Ancient Mariner" is the consequences of moral guilt. List quotes that support this theme.
This is an interesting theme, because in a sense, the suffering that the Mariner and his crew go through is not a result of the guilt of the Mariner but because of the "Polar Spirit" that the Mariner has angered through his act of killing the albatross. However, it is certain that although the Mariner is able to serve his "penance" for killing the albatross, this is an experience that burdens him for the rest of his life. Note what he says in the final part of the poem and how the Mariner finds himself compelled to re-tell his tale to find some sense of peace:
Forthwith this frame of mine was wrenched
With a woeful agony,
Which forced me to begin my tale;
And then it left me free.
Since then, although his experience is over, yet the after-effects linger on, for at unpredictable moments, a feeling of agony grips the ancient Mariner and will not go away until he recounts his experiences:
Since then, at an uncertain hour,
That agony returns:
And till my ghastly tale is told,
This heart within me burns.
Thus if you are going to look at the theme you have stated, I would investigate the impact of this narrative on the Mariner and how he is compelled to re-tell his tale again and again, thus showing the impact of moral guilt because of his actions, even though he had served his "penance."