In The Scarlet Letter written by Nathaniel Hawthorne Chapter 12, why does Dimmesdale respond to what he imagines with "grotesque horror"? Why does he suddenly burst in to "a great peal of laughter"?

In this scene, Dimmesdale has climbed onto the very same platform where Hester had to stand when it was first discovered in the community that she had committed sexual sin in the eyes of the Puritans. Dimmesdale, the father of Hester's child, has never been punished or publicly shamed. Here, he imagines that the whole town comes to see him standing there, revealing what he has done. The secret nature of what he perceives to be a deep dark sin is eating away at him, and so the dark fantasies of the truth coming to light are born from a twisted place. He is a Puritan preacher, making the contrast between the person he pretends to be and the person he feels like on the inside even more painful to bear. As this novel has many gothic elements to it, the use of the words like "grotesque horror" indicate his dark, sinful, wretched state of mind. And the creepy laughter further illustrates the idea that Dimmesdale's sin is consuming him, causing mania and hallucinations.