(goes back in time and kills Hitler and stops the Nazi party from gaining power in Germany)
Time Traveler: Whew. I've saved over six million people. Now to go back to the future.
(goes back to the future to see a radioactive glass desert of a planet and ruined cities coated in fallout)
Time Traveler: HOOOOWWW?
Random Sentient Robot: Yeeeah, see, those six million Jews you saved kinda multiplied over the next 80+ years and eventually all of Europe became Zionist. Then tensions flared between Zionist Europe and the China/Soviet alliance. The resulting nuclear war killed billions. Hope you enjoy eating roach burgers cause that's all we have to eat.
The lesson here kiddies? DON'T FUCK WITH HISTORY!
(Note: I am NOT SAYING those six million people deserved to die. I'm simply saying that messing with history would have, er, unintended consequences)