Question

What was the terrible deed done by the old Mariner? Why was it terrible?

Answer 1

The rime of ancient mariner explains about the deed of the mariner.
According to the fellow mariners, the mariner had killed the albatross which was considered to be good luck . They considered that, as the bird was shot, it brought them ill-luck which created unsuitable weather conditions.

With regards

Answer 2

He killed the Albatross,"The bird of good omen" . He thought the bird to be an ill omen and blamed it for all the misfortune.