What did the Mariner do to the albatross in 'The Rime of the Ancient Mariner'?

The Ancient Mariner tells the wedding guest in the final
stanza of the first part of the poem of his thoughtless act which has such cataclysmic
and bizarre consequences for him and the rest of the
crew-

God save
thee, ancient Mariner!
From the fiends, that plague thee thus!—

Why look'st thou so?”—With my cross-bow
I shot the
Albatross!

The mariner then
tells that he is punished by the rest of the crew for his mean and portentious act.

And I had
done an hellish thing,
And it would work 'em woe:
For all averred,
I had killed the bird
That made the breeze to
blow

In a bid to assuage the
evil spirit which the crew believe is now plaguing the boat, the mariner is made to bear
the burden of the killing of the albatross alone -

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Ah! well a-day! what evil looks
Had I
from old and young!
Instead of the Cross, the Albatross
About my
neck was hung.