When considering the stories that delve into the nature of insanity and track down the slow descent of their characters into madness, The Tell-Tale Heart is, perhaps, the most famous one and by far the darkest of them. The novel shows different levels of insanity, exploring how the lead character can lose their sanity. The process of going insane described in the novel delves into the darkness of human nature, thus, exposing its deepest fears.
It could be argued that the lead character questioning his sanity at the very beginning of the novel can be seen as one of the signs of his madness. The insanity aggravates once the lead character starts to design plans to kill the man whom he finds so abhorrent. The moment at which the lead character admits that he is literally possessed by the idea and that it haunts his every step indicates that he is starting to descend into madness. Afterward, the novel pushes the envelope of portraying the sick mind of the lead character by describing him murdering the old man. However, the tension does not reach its climax here and, instead, peaks at the end of the novel as the lead character starts hearing the heart-pounding beneath the floor.