Suppose one could go back in time and interact with the past. Would you be prepared to go back in time to prevent a terrible wrong knowing that doing so destroys you and world of your present time. Almost everyone alive today was born after Hitler took power in Germany. If you could go back in time and kill Hitler, would you do it? Think about the consequences. The world you know and everyone in it would cease to exist in the new future you created. You save the 50 million lives lost in WW2, perhaps, but you condemn to non-existence the billions alive today. Is that a fair trade-off? Would you want that responsibility?