Feel a glory in so rollingOn the human heart a stone-- (line 84-85)
Here's where the "happiness" in this poem takes a turn for the creepy. Now we're talking about the joy and the "glory" that the ghouls feel when they spread misery. Think of these guys as being a little bit like the Grinch. For them happiness is misery, and misery is absolute joy. They turn everything upside down. Just like the Grinch, they perch up above the town and chortle when they see signs of sadness.
And his merry bosom swells (line 93)
The king of the ghouls is the meanest and nastiest creep of them all. When he rings out death and suffering over the town, his heart fills with delight. This is the dark mirror image of the more "normal" joy that we saw earlier in the poem. In fact, that's kind of how the whole poem works, like a light and a dark reflection of the same story.