The blood of Lupito mixing with Narciso's blood symbolizes those who have been killed and those have been forgiven. This foreshadows Andrew being forgiven in the near future.After the dream, Antonio will forgive those who have wronged him and anyone.
Forgive Narciso please!
The dream Antonio had symbolizes that he is human and is still flawed like everyone else. His desire to punish Tenorio has foreshadowed that type of person he'll become. Not only that, but the dream symbolizes the guilt, hatred, and fear built up inside of Antonio.
The people decaying symbolizes all the sinners who have not yet been forgiven. This foreshadows how Antonio will change in order to save all of these people from their sins. This horror of a dream is going to make Antonio see what life is like and how many sinners crawl around Earth.