The original Star Trek did an excellent episode on that possibility, which is anyone who traveled back in time could not alter any events without running the risk of affecting their own existence.
It might explain why if there is in fact those from the far future who managed to go back have done nothing to prevent a Hitler or a Stalin.
The Back To The Future movies were great. It discussed alternate time lines, alternate worlds. The Terminator, which at first seemed like just an action flick, was actually a great science fiction story discussing time travel. The Quantam Leap television series was very good and often poignant. I remember reading a sci-fi story - I believe by Joseph W. Campbell - that discussed looking into time. But the further ahead you looked the blurrier it got because of the different things that could happen from simple yes/no decisions. I read another sci-fi story years ago called Night Eyes. It seemed to be about alien abductions but what it turned out to be was that the 'aliens' were actually humans from the far future. They 'abducted' people from the same family line in order to stop an event that evolved them into the 'grays.' Does anyone remember the television program The Time Travelers from the sixties?