For Coleridge, the source of guilt is a combination of his mental disorder and his addiction to laudanum and opium. In one of his letters to Joseph Cottle, dated April 26, 1814, he writes:
. . . for ten years the anguish of my spirit has been indescribable, the sense of my danger staring, but the consciousness of my GUILT worse, far worse than all. I have prayed, with drops of agony on my brow, trembling not only before the justice of my Maker, but even before the mercy of my Redeemer. "I gave thee so many talents, what hast thou done with them?"
Coleridge recognized his own mental imbalance. Given that he was a devout Christian, this infirmity challenged his faith. Why would God give him this gift of poetry while plagued with this mental distress? So, part of his source of guilt is that his mental disorder allowed him to occasionally doubt his faith. The other, perhaps main, source of his guilt is his addiction to opium. He notes in this letter that he had freely admitted to friends of this "ACCURSED habit."
Note that Coleridge suffered this guilt for years. Likewise, the Mariner suffered his guilt for years after the incident at sea. One might compare Coleridge's mental disorder with the Mariner's killing of the albatross; and the completion of this analogy would be the comparison of the Mariner's penance of repeating his story with Coleridge's repeating drug use.
The Mariner's shipmates scold him for killing the albatross. Coleridge's friends, while understanding his mental state, may have pleaded with him to quit the drugs. The shipmates could not understand the Mariner's action. Those around Coleridge could not really know how mentally distraught he was. Both Coleridge and the Mariner suffered guilt for something they did haphazardly. The Mariner shoots the albatross seemingly for no reason. In that same letter, Coleridge notes that his addiction began in ignorance; he used it to cure his swelling knees.
In the poem, there is no precursor, no real reason as to why the Mariner shoots the bird. Coleridge began using laudanum to cure a leg problem. But both men realize, eventually, that their actions led to feelings of guilt.
And I had done a hellish thing,
And it would work'em woe:
For all averred, I had killed the bird
That made the breeze to blow. (91-94)
Killing the albatross interrupted the ship's journey and eventually altered the Mariner's life. Coleridge's mental imbalance and subsequent drug use also altered the course of his mental life.