Why do the mariners change their minds about the value of the bird?

 The other sailors were angry with the Ancient Mariner for killing the Albatross, which they believed had saved them from the icy world by summoning the wind: "Ah wretch! Said they, the bird to slay / That made the breeze to blow!" Then the mist disappeared and the sun shone particularly brightly, "like God's own head." The sailors suddenly changed their opinion. They decided that the Albatross must have brought the mist, and praise the Ancient Mariner for having killed it and rid them of the mist: "Twas right, said they, such birds to slay, / That bring the fog and mist."