You, A Time Traveler?

Some time ago, I read Stephen King's 11/22/63, a book in which Jake Epping, a thirty-five-year-old high school English teacher comes upon a time portal which can take him back to the 1960s. Among other things, he decides to take on a mission to stop the assassination of John F. Kennedy.
It got me thinking about the ethics of such an endeavor, if were possible. If you could actually go back in time and stop something from happening, it could potentially save lives, prevent atrocities, etc. But then, who knows what the far-reaching effects altering the timeline would be? Would the world as we know it exist when we return from our mission? Would it be better, worse, unchanged?
Given the risks, would you still embark on such a mission if you had the power to do so?
If I had the power to go back in time and stop one disaster or atrocity from happening, it would be the Massacre of Nanking in 1937. Yes, there have been other such genocidal war crimes, but this one was particularly barbaric, considering what they did not only to the hundreds of thousands of innocent women, but to children and infants.
So, if there was one disastrous event you could go back in time and prevent, what would it be? Please discuss in the comments section at the bottom of this page.