But whence proceeds this moral Evil? Why was not Man formed without the capability of it?

Coleridge, in a lecture of 1795

Woe to the man to whom the origin of evil is an uninteresting question

Coleridge, notebook, 27 October 1803

A year before Coleridge started composing The Ancient Mariner, he listed twenty-seven works he intended to write. Among them was a lengthy verse he titled The Origin of Evil, an Epic Poem. The subject had fascinated Coleridge from an early age. It seized him after the French Revolution ended in bloodshed and all hope of social renewal in France and in England were nipped in the bud. As a young man, he defended social injustice. At the age of twenty-three, he gave a series of lectures in Bristol in which he criticised the privilege of large landowners, the curtailment of free speech and the slave trade.

Roots of evil
Coleridge wanted to locate where the roots of evil in man lay - in others collectively and individually, and in himself. He was acutely aware of his own failures. This epic narrative poem, he thought, would settle it. He intended to take his time as evident from the following letter to his publisher:

I should not think of devoting less than 20 years to an Epic poem. Ten to collect materials, & warm my mind with universal Science – I would be a tolerable mathematician, I would thoroughly know  mechanics, hydrostatics, optics, & Astronomy – Botany, Metallurgy, fossillism, chemistry, geology, Anatomy, Medicine – then the mind of man – then the minds of men – in all Travels, Voyages, & Histories. So I would spend ten years – the next five in the composition of the poem – & the five last in the correction of it. 

Of the twenty-seven works that Coleridge envisioned, he wrote none - not even The Origin of Evil. He could not concentrate sufficiently for an in-depth study in which he would incorporate all knowledge of all the fields he listed. His life was too chaotic for that, his discipline too weak. Moreover, due to his opium addiction, he lacked the physical and mental strength.

Cain and Abel
An anecdote that Coleridge himself relates of his failed attempt at another joint project with Wordsworth also testifies to his preoccupation with the origin of evil. Just before starting on The Ancient Mariner with his friend, during the autumn walk through the Somerset hills, he suggested writing a poem about the first brothers in the Old Testament, Cain and Abel. The poem was to be titled The Wanderings of Cain and consist of three parts. They agreed that William would write the first part, he would write the second. Whoever finished first would then take care of the third part. In his own words, Coleridge hurled his share on paper 'at full finger-speed'.  He related thirty years later that when he went to show his manuscript to William, he found his friend with:

a look of despondency fixed on his almost blank sheet of paper, and then its silent mock-piteous admission of failure struggling with the sense of the exceeding ridiculousness of the whole scheme – which broke up in a laugh: and the Ancient Mariner was written instead.

Child of wrath
The Origin of Evil got stuck in ambition and hubris. The Wanderings of Cain stranded in a futile attempt at collaboration. Immediately after this last failure, the six hundred and sixty lines of The Ancient Mariner poured out of his pen in four months. It seems as if here, in the apparently senseless manslaughter of an albatross, he finally found a river for his poetic imagination to flow through. But with a poem you cannot solve a mystery, not even the mystery of the human condition. A poem may perhaps, like a nightmare, express the dark sides of the human soul in images, but it does not provide a rational explanation. The mystery continues to haunt Coleridge throughout his life. In 1803 he still writes: 'I made out however the whole business of the Origin of Evil satisfactorily to my own mind'. But in his Confessio Fidei (Confession of Faith) seven years later, he acknowledges that he does not understand the 'fearful mystery':

I believe, and hold it as the fundamental article of Christianity, that I am a fallen creature; that I am of myself capable of moral evil, but not of myself capable of moral good, and that an evil ground existed in my will, previously to any given act, or assignable moment of time, in my consciousness. I am born a child of wrath. This fearful mystery I pretend not to understand. I cannot even conceive the possibility of it, - but I know that it is so. My conscience the sole fountain of certainty, commands me to believe it, and would itself be a contradiction, were it not so - and what is real must be possible. 

We are born in sin. Evil clings to us. This Christian conviction had been instilled in him, son of an Anglican minister, since birth. But he was much less strict in faith than his father. An irrepressible counter force constantly beleaguered his orthodoxy. In the crevices of his mind, the spectre of atheism was always haunting and seeking its escape, as it did in The Ancient Mariner