This quote is actually the final line of this tremendous Gothic short story from Edgar Allen Poe. It is uttered by the anonymous first-person narrator, whom we come to suspect is profoundly unreliable, and is his confession of the murder he has committed. To put this quote into context, you really need to read the two or three paragraphs before this final paragraph, which describe how the narrator sits the detectives down right over where he has hidden the corpse of the old man and talks to them, rejoicing in his genius and his intelligence at having outwitted them. However, as the conversation continues, he hears a sound that becomes increasingly louder, which he comes to believe is the beating of the old man's heart. He suspects that the detectives are just pretending not to hear it as they watch him become ever more erratic:
Was it possible they heard not? Almightly God!--no, no! They heard!--they suspected!--they knew!--they were making a mockery of my horror!--this I thought, and this I think.
Thus it is that at the end of the story, the narrator, driven to distraction by the sound and by the way that he feels he is being mocked, utters the quote you have picked out, calling the detectives "Villains" and asking them to "dissemble no more." Of course, whether we believe the narrator or not in what he asserts is another matter.