Travis kills Eckels at the end of the story because Eckels disobeyed the rules of traveling back into time to the dinosaur age—but as we will see, Bradbury implies it is more complicated than that.

Eckels panicked on the safari to the dinosaur age and left the special path constructed to safeguard against any interference with the past that could alter the course of history. In the process, he stepped on and smashed a butterfly: a seemingly minor act, but one that changed the future in profound ways.

We know the future is different when the safari party returns because common English spellings have changed, and more importantly, now Deutscher, not Keith, has won the recent presidential election. As a worker says:

We have got an iron man now, a man who isn't afraid of anyone or anything!"

The piece on information that a would-be dictator has been elected, along with knowing he is responsible for it, anguishes Eckels. Eckels lets out a cry and falls on his knees. This complicates Travis's possible motivations for killing Eckels: the sequence of events suggests that Travis kills Eckels as much out of mercy—pity for his guilt and agony—as for punishment.

Ah, but does Travis kill Eckels? We only know that the final “sound of thunder” is probably a gunshot. Travis could kill Eckels. Or he could kill himself. Obviously, Eckels accidentally changes the future when he steps off the path and onto a butterfly. He has been told the rules and the need for them to be followed. But the dinosaur so terrifies him, that he is lucky enough to even make it back to safety of the time machine. The group cannot go back and repair the damage he has done. And now people have to live with a dictator as a president, instead of a compassionate leader. Someone has to pay the price for the mistake. If Travis kills Eckels, he’s issuing the ultimate penalty for the man’s blunder. If Travis kills himself, he won’t have to live under this new regime, and he won’t have to answer to authorities about the fault of Time Safari to use time travel wisely. But Eckels, in turn, would have to live with his mistake. Which fate would be worse?