Why did the Mariner kill the albatross?

The ancient mariner sails away together with the rest of the crew. There is this bird, the albatross, that goes together with the sailors and helps them move all along. It's a bird of good omen. Suddenly the Mariner shoots the albatross and everything changes, the rest of the sailors are doomed and so is the mariner who will be chased for eternity by the spiritis of the supernatural world...

Why would the Mariner have done something like that? Why did he kill the Bird of Good omen?