Why would "The Tell-Tale Heart" be particularly disturbing to Victorians?

The Victorians were particularly concerned with duplicity and deception and a person's ability to be secretly sinful or suffering from "madness" while tricking the world into believing that they were morally upright and/or mentally sane.  The thought that one could be fooled by such a person and exploited, injured, or otherwise harmed, was both a frightening and titillating prospect.  Therefore, they enjoyed fiction that focused on characters who enacted this kind of fraudulent behavior because they found such persons so disturbing.

The narrator of "The Tell-Tale Heart" most definitely qualifies as duplicitous.  He says that he "was never kinder to the old man than during the whole week before [he] killed him."  By day, the narrator appears to be friendly and loving to the old man (though his relationship to the old man is never clarified: the old man could be his father, a roommate, etc.), but, each night, he spends spends hours practicing the movements he will use to kill the old man when the conditions are just right.

Further, after he's committed the murder, he is such a powerful dissembler that he manages to fool the police officers who came to investigate the scream his neighbors reported hearing.  The narrator says, "The officers were satisfied.  My manner had convinced them."  It isn't until his own paranoia and delusion drive him to confess that they have any sense that he is guilty.

Such a powerful deceiver, one capable of inflicting such damage without scruple, one who actually believes that his actions are justified, would have been both particularly interesting and particularly frightening to Victorian readers.

What makes the villain in "The Tell-Tale Heart" disturbing?

There are several elements that make Poe's antagonist particularly unsettling and disturbing. The unreliable narrator of Poe's classic short story "The Tell-Tale Heart" is depicted as a neurotic, mentally unstable man, who claims that he is perfectly sane. However, in the first paragraph of the story, there are several red flags that suggest he is mentally deranged. The narrator's staccato, fragmented sentences, his insistence on being sane, and his ability to hear "all things in the heaven and in the earth" indicate that he is mentally deranged. The narrator proceeds to mention that he loves the old man and says that the old man's vulture eye is his sole motivation for murdering him. The incongruity between the narrator's feelings and actions once again reveals his mental instability. The sadistic thoughts and motivations of the narrator are significantly disturbing.

In addition to the narrator's deranged mind and confounding motivation, Poe’s antagonist proceeds to sneak into the old man's room when he is sound asleep for eight consecutive nights. Audiences find this method particularly unsettling because they can identify with the helpless, vulnerable old man. Simply the thought of a malevolent, deranged individual spying on you as you sleep is both disturbing and nerve-racking. The narrator then details how he suffocated and dismembered the old man's body before finally succumbing to his guilt and anxiety when the police arrive and begin questioning him. Overall, the narrator’s mental instability, his unsettling method of spying on the old man, and his horrific murder are what make Poe’s antagonist such a terrifying, unforgettable character.