The mariner does not explain exactly why he shot and killed the albatross with his crossbow. When the albatross first came to the ship, the sailors thought it was good luck, for a good wind followed it. Thus, when the mariner shot it, they were initially angry. They eventually changed their minds, thinking he was right to have shot it; but after days with no wind, they began to blame him again.
Though the mariner's motives for killing the albatross are not clear, perhaps he wanted to show the other sailors that no bird could be so powerful—that their fate was not determined by something as simple as a bird. He may have wanted them to see that it is men who are powerful, not forces of nature.
This poem was composed during the Romantic period of poetry. Writers of this period were concerned with the relationship between people and natural forces. They often used themes that emphasized the idea that people do not control the natural world. Instead, they are at its mercy. By killing the albatross, it may be that the mariner was trying to raise himself up to be more powerful than mortal men can be. The ill fate that befell the ship as a result of his foolish action indicates that no one can rise above the forces of nature. Instead, they must live in harmony with the natural world if they hope to receive its blessings.