Lawrence attributes human and even godlike qualities to the snake, calling it his "guest," saying that the snake "mused a moment," and imagining that it is seeking his "hospitality." The reptile is moreover described as turning his head in "as if thrice in a dream." It looks around "like a god, unseeing, into the air."
The speaker both fears the snake and, although he knows it's dangerous, seems to pity it as well, and then regrets his clumsy attempt to kill it by throwing a log at it. He then likens the snake to the albatross, presumably of Coleridge's Rime of the Ancient Mariner, which the mariner needlessly kills, and in doing so brings misfortune upon himself and his shipmates. The speaker regards the snake as having even further human qualities in believing that it feels a "sort of horror" at having to return to the darkness of the earth after coming up to drink at the water trough.
Lawrence's theme, like Coleridge's, is probably that of a wish for human solidarity with the animal kingdom, and with all life on earth. If a dangerous reptile can be sympathized with, if it has feelings (to put it simply) like those of human beings, then it is wrong to kill any living thing.