Question

what hellish thing had the mariner done?

Answer 1

The hellish thing that the mariner had done was that he had killed the Albatross. Everybody thought that it had brought them good luck and charm and everyone on the ship played with it and gave it food too. So by shooting it down the mariner had done a hellish thing !

Answer 2

he just killed the bird

it was hellish because he had done it without a cause.

the was innocent and friendly with all others and it did not created any circumstances that led to the birds death .