"Determination becomes obsession and then it becomes all that matters."
-- Jeremy Irvine
Poe presents the narrators of "The Tell-Tale Heart" and "The Cask of Amontillado" as devious, obsessed characters. Both are overpowered by the need to consume the life of their victim. Though they use different strategies to carry out the murders in different ways, obsession is the driving force in both. It is this obsession that inspires them to design cunning strategies and carry out the executions.
The obsession of Montresor in "The Cask of Amontillado" and of the narrator in "The Tell-Tale Heart" is obvious throughout the stories. The narrator in "The Tell-Tale Heart" is actually obsessed with the old man's eye, rather than the old man himself. It is this obsession with the eye that drives him to commit the murder, despite his relatively good feelings toward the old man personally. This is why he is unable to harm the old man when the eye is concealed. His obsession with the eye is what controls him and his actions. Without it in sight to enrage this obsession, he is unable to harm the old man. This also is why he must shine the lantern light upon only that eye. By leaving the rest of the old man in the dark, he in a sense de-humanizes the victim. His obsession intensifies and takes full control of his actions. He eliminates the old man from the equation and is able to charge him and make the kill.
Montresor in "The Cask of Amontillado" is similar to the narrator in "The Tell-Tale Heart" in that his obsession with consuming the soul of Fortunato influences his every action. However, it is with Fortunato himself that he is obsessed. He feeds off of Fortunato's pain, unlike the narrator in "The Tell-Tale Heart" who's obsession is with destroying a menacing inanimate object. Montresor's entire conspiracy is focused around making Fortunato suffer, and for him to know just who is causing this suffering. This is why he goes to such lengths to put together this intricate strategy. It could have been so much easier to kill Fortunato in some easier, quicker way. Instead, he dedicates himself to torturing Fortunato. He creates a plan that leads Fortunato into the depths of the catacombs beneath his home, and kills him in an excruciating manner.