what quote that can prove that the narrator killed the old man?

"The old man's hour had come! With a loud yell, I threw open the lantern and leaped into the room. He shrieked once --once only. In an instant I dragged him to the floor, and pulled the heavy bed over him. I then smiled gaily, to find the deed so far done. But, for many minutes, the heart beat on with a muffled sound. This, however, did not vex me; it would not be heard through the wall. At length it ceased. The old man was dead."