"I shot the Albatross!"

In the time of the ancient mariner, the mariner shoots an albatross after it brings fortune to the crew. However,  reason is unstated and unclear in the story. I believe that the mariner shoots the bird because the fog and snow that surround the ship blind the mariner. Also, he may be afraid that the albatross is a coming iceberg that can sink the ship. Even though the write didn't directly state the reason why the mariner kills the albatross, the mariner eventually learns his lesson and is forgiven for his sin.