Many of the characters and plots in Poe's short stories involve psychologically disturbed people. This story is a good example, and perhaps the most violent of them all. Its protagonist, Egaeus, is a deeply disturbed individual, a monomaniac, a type of mental/personality disorder which Poe was fond of employing in many of his tales of gothic fiction.
Egaeus is supposed to marry his cousin, Berenice, even though she is physically ill and frail. He is mentally ill. He fixates on objects and exhibits other characteristics of a mentally ill person such as obsessive-compulsive disorder, trance-like behavior that he does not seem able to control, bouts of depression and euphoria typical of manic depression, etc. One of the things he fixates upon in this story is Berenice's teeth. After Berenice supposedly dies and is buried, in one of his disassociative states, Egaeus somehow digs up her grave and extracts all 32 of her teeth with dental instruments. The reader never sees this happen, but from Poe's description of what happens, the reader infers that Egaeus has pulled out Berenice's teeth without realizing she was buried alive. His clothing is covered with mud and blood and these images lead the reader to infer what happened.