In "The Tell-Tale Heart," the terror is compounded by the nature of the conflict. The conflict which exists in the story is both internal (based upon the narrator's diminished mental capacity) and external (based upon the hatred of the old man's eye). While many people may be able to relate to despising an attribute of another persons, they (assumedly) are not able to relate to the murdering of one because of a detestable attribute.
Therefore, terror is compounded not only from the imagery Poe provides in the story, but also because of the actions of the narrator. It is one thing to be afraid of something such as the physical attribute of another; it is far more terrifying to consider a hatred so great that murder must be committed.
Based upon this, the terror is compounded by the nature of the conflict being so unidentifiable that horror typically fills the mind of the reader. readers typically would question the fact that how can one hate an object so much as to want to destroy it forever. Because of this conflict, exhibited by the narrator and the reader, terror is certain.