What is the sound of thunder at the end of "A Sound of Thunder"?

The first sound of thunder that Eckels hears are the steps of the gigantic Tyrannosaurus Rex walking through the forest. The thunderous steps drown out the myriad of forest sounds as Eckels and the hunters prepare themselves to shoot the majestic beast. However, Eckels becomes frightened and runs off the Path, accidentally stepping on a prehistoric butterfly, which dramatically alters human history. Once the time travelers return to the Time Safari office, Travis and Eckels realize that history has been altered and are astonished to discover that Deutscher is the current president. Eckels then drops to his knees and pleads for Travis to travel back in time in order to rewrite history once again. Travis then shifts his rifle, takes the safety off, and raises his weapon. The last sound of thunder is the noise from Travis's gun once he fires his weapon. The story's ending is ambiguous and the reader can argue whether or not Travis commits suicide or kills Eckels for his tragic mistake.

The first time we hear this phrase is when the dinosaur crashes into the jungle. The noise it creates is like the roar of thunder during a tremendous storm. It scares Eckels enough that he has to hurry back to the time machine.
The second time the phrase appears is at the very end of the text. The time travelers come back from the past only to discover that the course of history has somehow changed. Travis takes up his rifle in order to right this wrong. Bradbury ends the story with the quick and ambiguous note, “A sound of thunder.” What really happens here? Evidently someone gets shot, and the acoustics cause the impact to resound throughout the office. We are left to wonder. Does Travis kill Eckels, or himself? A solid case could be made for either alternative. He could have killed Eckels for stepping on the butterfly. Alternatively, he could have killed himself in order to absolve himself of the responsibility for the accident leaving Eckels to live with the changes he has inflicted upon the world. What do you think?