"The Rime of the Ancient Mariner" conveys the theme that humans will suffer until they develop the humility to understand that all of nature is God's creation and must be treated with love and respect. Coleridge, in a sense, could be seen as having the sensibility of a climate change activist ahead of his time, preaching harmony with the natural world.
The mariner's great crime is shooting the albatross even though the creature has done no harm—and has, in fact, been helpful. The mariner kills it thoughtlessly, not with any malice, but just because he can. He does so because he is the human; he has the power. This great mistake brings a curse to bear on the mariner and his shipmates.
It is only when the mariner sees and thanks God for the beauty of the water snakes that he finds redemption. He says of these lovely creatures who are blue, green, and "velvet" black :
O happy living things! no tongue
Their beauty might declare:
A spring of love gushed from my heart,
And I blessed them unaware
This blessing lifts the curse on the mariner and his men. They are freed from suffering, but the mariner carries the burden of telling his story and bringing his message of the divine grace of nature to as many people as possible.
The mariner moves from a position of believing that he has dominion over the natural world to developing humility and awe over it as a manifestation of God.