A man tells him he's a murderer—how can a single sentence unmake a person?

The man did not look at him.

“What do you mean... what is.... Who is a murderer?” muttered Raskolnikov hardly audibly.

“You are a murderer,” the man answered still more articulately and emphatically, with a smile of triumphant hatred, and again he looked straight into Raskolnikov’s pale face and stricken eyes.