The 'Heart of the Monster Story' is often about the human - monster relationship. It's about how humans create monsters, either literally like Dr. Frankenstein creating his creature, or metaphorically by their actions and vices. The emotions that the monster evokes in the characters and the readers, such as horror, pity, or fascination, are also part of this heart. It is the very thing that gives life and meaning to the narrative about the monster.
Well, the 'Heart of the Monster Story' could be different things. It could be the motivation of the monster. For example, in Frankenstein, the monster's loneliness and search for acceptance can be seen as part of the heart of the story. It could also be the way the monster challenges our perception of normal and abnormal, good and evil. This contrast and exploration are often at the core of such stories.