If you could go back in time and change a historical event would you ? Now I may be sounding like H.G.Wells here, but just for the moment imagine that you could go back in time and return to the time that you had left. Well, if you interrupt a historical time line then when you return the present will have been affected by what you did to a lesser or a greater extent. And all the more so if what you changed was of great historical importance.
My brother told me about a story by Stephen King in which a time traveller went back to Dallas, Texas on the day of John F. Kennedy's assassination. Kennedy was not killed through the efforts of the traveller - he survived and the traveller returned to the day that he had left and found the world devastated by a nuclear exchange. I think when we think of the possibility of time travel and changing terrible events we assume that the change will be for the better, but it can just as easily be for the worst. Food for thought.