Both of the stories are told in first person narration. The main characters of the stories are both murderers who are "tormented" by something and driven to murder because of it. In "The Black Cat," the narrator is an overly sensitive man who was always teased as a boy, and as he got older became an alcoholic. He abused his wife and cat, and kills the cat while drunk. In order to replace this cat, he gets another one, but this cat reminds him so much of the cat he killed that he can't stop thinking about it. Even the markings on its body look like a hangman's noose. It torments him until he tries to kill it, but his wife stops him so he turns on her and kills her. The cat reveals to the police where the body is; the narrator believes the cats in this story constantly "torment" and bring evil upon him, even though his downfall is his own doing.
In The Tell Tale Heart, the narrator is also tormented by an old man's cloudy eye, and it "torments" him so much that he kills the old man. The narrator then hears the dead man's heart beating, beating, beating, making him feel guilty and driving him mad with the sound. The beating heart that he believes he hears eventually makes him reveal his guilt to the police.
In both of these stories, there is a psychological terror in the minds of the narrators. Each have an outside force that they believe is driving them to murder, and driving them insane. Each of these forces is also their undoing.