Well, for this you have to read the poem carefully as there are some specific lines in the poem in which Coleridge points to the despair, which mariner went through. The fact of killing a bird, slaughtering albatross, casts heavy stress and despair in the mariner. You can see a despairing reflection upon this action, that reads,
“And I had done a hellish thing,/And it would work ‘em woe.”
His experience of despair appears in the imagery of “work ‘em woe,” and “a hellish thing.” After doing such an action, mariner expresses his despair by saying that, “I had killed the bird.” You can see his further condition of despair is evident in this statement, “Twas as sad as sad could be.”
In all these moments, mariner realizes that what has done and its implications illuminate the meaning of this poem.