"Thanos did nothing wrong" has become one of the most common mantras on the Internet in recent days, and it just sparked one of the biggest events in Reddit history, but most people still don't understand what all of the commotion is about.  So let me try to break it down very simply. 

In the most recent Avengers movie, the story centers around a super-villain named Thanos that intends to wipe out half of all life in the universe.  He does not want to do this just to be evil, but rather his plan is to get population growth under control so that those that remain will be able to enjoy happy, sustainable lives. 

If that sounds uncomfortably close to something that you have heard before, that is because it is.  Population control is a major theme on the radical left, and many of them truly believe that humanity's population must be greatly reduced "to stop global warming" and "to save the planet".  In the film, Thanos truly believes that he is doing the right thing, but since he is the villain everyone in the audience is theoretically supposed to be rooting for him to be defeated.  But instead of being universally hated, Thanos has become the big breakout star from this movie.  Large numbers of people are insisting that "Thanos did nothing wrong" and are embracing his population control philosophy.