]If time travel is possible, it makes it so that you can't change anything in the past, because the past already happened. Like, if you met your future self and had a conversation with him, it's inevitable that you would eventually go back in time after that and have that same exact conversation with your past self, but the roles will be reversed. Or, if you saw the episode of the Twilight Zone where a guy goes back in time to shoot Hitler, but before he can do anything, he is caught by the Gestapo and he fails to make a difference in history. In your case, it's impossible to kill your grandmother anyway, because even if you try, she will still survive and you will be born, because even before you went back in time, your grandmother kept it a secret that someone who looked just like you tried to kill her a long time ago. Does that make more sense?