Ahhh I remember Tell Tale Heart. I think we did a unit on it in middle school... Edgar Allen Poe makes some great short stories :)
Anyway, The narrator in tell-tale heart is not a reliable one. He's not well in the head, if you know what I mean. The author drops clues in the story that readers can pick up on to support this claim. He tries to reassure readers that he is not crazy - at the beginning of the story - but his actions suggest otherwise. For example, he admits that he doesn't even know why he wanted to kill the old man, and blames it on his "evil eye". We also see that he gets easily anxious and paranoid, from how attentive he is to his surroundings. The narrator even hallucinates that he could hear the beating of the old man's heart... AFTER he died.
So yeah, he's a little cuckoo