Why does the Mariner kill the albatross? What is the symbolic nature of the action?
The Mariner kills the albatross because he associated the lack of wind with it. At first all the men thought the bird was good luck since a good wind blew and they moved swiftly. Then, the wind died and they blamed the bird. THe sailors cheered when the Mariner killed the bird which is symbolic of animal abuse. By killing the bird, he is disrespecting all of nature--a sin since the poem states: all creatures great and small the lord God created them all.
Once the Mariner "blesses the snake unaware," then he begins the long trek back to being forgiven and living out the rest of his life wandering the earth and teaching others how to treat mother nature and all her creatures.
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The killing is definetely no 'impulsive act'.
Look up for cross (mention the symbolysm, by the way...)-bows. At the supposed timeset for the journey (at about 1500) simple shotguns were already invented. Moreover, the act of loading a cross-bow needs strength and time as it never has been carried loaded.
I wouldn't agree with amy-lepore, because the mariner was blessed as well as punished by the other seamen for killing the bird.
So I agree with thedra: killing the bird isn't central; it's what follows (even if birds are creatures who -by nature ;-) - live between earth and heaven).
But what follows isn't a 'godly punishment'. No. Nature is spiritualised before 'God' comes into play.
Does 'nature' punish the mariner? Does 'God' punish him? Does he punish himself?
To answer your question: the symbolic nature of the action is that one could regret what one has done before.