The wedding guest is "sadder and wiser." Possibly, he is sadder because he himself is in need of repentance. Perhaps, the wedding guest has prejudices against some of God's creation that some consider lesser than.
Although the mariner kills the albatross, he has now learned his lesson and feels that all of God's creation should be respected. In fact, that is what he tells the wedding guest before his departure:
Farewell, farewell! but this I tell
To thee, thou Wedding-Guest!
He prayeth well, who loveth well
Both man and bird and beast. (lines 611-614)
The wedding guest has learned so much from the mariner. He realizes how much the mariner has suffered for killing the albatross. Perhaps, the wedding guest is much wiser for hearing the story. Perhaps, the wedding guest has a new appreciation for all of God's creation. An albatross is a clumsy bird on its feet. Perhaps, it represents those who are significantly different and viewed by some men as less graceful. Perhaps, the wedding guest is sadder because the mariner has had to suffer so. Perhaps, the wedding guest has hidden prejudices that are unresolved. However, since the wedding guest is wiser, he will surely make the error of his ways right. There is a reason why the wedding guest becomes engrossed in the mariner's story. Perhaps, he could associate with the mariner in his prejudices of the albatross whom some have considered as a clumsy, lesser than bird.