When he is in Sicily, near Mt. Etna, the speaker goes to a water trough to fill his water jug. There, he is startled to see a snake. He debates whether or not to kill it, as it is a poisonous snake, and he debates whether his refusal to kill it is an appreciation of its beauty—he writes, "How glad I was he had come like a guest in quiet, to drink at my water-trough"—or merely cowardice. Finally, he throws a log at the snake, causing it, although it is already leaving, to wriggle off in awkward haste. The speaker then feels ashamed at having done that, calling it a "petty" act.
The theme or message Lawrence is trying to convey is that the snake is a beautiful creature, one to be honored, not killed, for it is more in harmony with nature and more truly itself than an over-educated human being.
Lawrence uses positive language to describe the snake in a way that humanizes it. The alliteration of the repeated "s" sounds also conveys some of the feeling of a snake:
He sipped with his straight mouth,
Softly drank through his straight gums, into his slack long body,
When he throws the log at the snake, the speaker feels shame and a sense that his superior education is actually a curse:
I despised myself and the voices of my accursed human education.
Lawrence evokes Coleridge's The Rime of the Ancient Mariner when he alludes to the albatross the mariner thoughtlessly killed. Lawrence here communicates that he understands the snake as a blessed creature of God, to be honored and cared for:
And I thought of the albatross
And I wished he would come back, my snake.
He decides that attacking the snake, even if half heartedly, is an unworthy act, for a snake should be respected:
I have something to expiate: