Yeah, that's why I believe if time travel were possible it means we are living in the best possible world. What a horrifying concept. Think about it, if the bad stuff could be changed then they've (time travelers) have alread changed it to the best outcome.
The only other outcome of time travel would be absolute tyranny (if you think we currently are in a situation of absolute tyranny then you don't know youur history or current events).
-Blast, going to sleep via a very nice brandy...