If time travel to the past were possible it would likely happen in one of two ways. The first is the multi-vese way wich would work like this: if one were to go to the past and kill one of their grandparents they wouldn't really kill their grandparent but someone that was the same from another universe. That universe would develop differently than the one your used to. The second way would be the consistent way: If you went back in time you wouldn't be changing the past you would be fulfilling it. For example if you went back in time and tried to kill your grandparent you the gun might misfire or you might miss and hit JFK.