Time travel sounds like a grand idea at first. Traveling backwards and forwards like the main character in Dr. Who looks like fun, but is complicated by a whole universe of ethical considerations. Its not just as Barjavel had pointed out that someone could go back to the past and kill their grandfather, but it could be some action that seems relatively harmless which could seriously alter history unless you take the notion of parallel universes to be a serious possibility. For instance you could go back in time and strike up a conversation with an ancestor of Lincoln, Beethoven, or Einstein and what if your conversation kept them from meeting their future spouse and then there would be no Lincoln, Beethoven, or Einstein. Ultimately even if time travel does turn out to be possible, the best one could hope for in such cases would be to avoid these types of ethical problems and to only go back as an invisible observer of past events. So in reality the time machine would also have to be a cloaking device as well.
There maybe a possibility that would prevent any changes to the future by a time traveler. This is outlined as Novikov's self consistency principle: http://en.wikipedia.org/wiki/Novikov_self-consistency_principle. If that turned out to be true then it would be impossible for someone to go back and kill their grandfather, or to prevent Lincoln's ancestor from meeting their spouse.
In reality we are likely a long way away from developing time travel if it is even possible, but the risks should be weighed carefully when and if it is developed.