For years, Thanos has tried to obtain all six Infinity Stones, and with his subordinates and allies failing to gather them on his behalf, the Mad Titan decided at the end of Avengers: Age of Ultron that he'll get the job done himself. This is finally come to a head at the end of the month in Avengers: Infinity War, and recently the movie's previews has made it clear that with all six Stones in his Infinity Gauntlet, he plans on wiping out half the universe so that "balance" can be achieved. As for why Thanos wants to do this, it all stems from him failing to save his homeworld from extinction with a brutal plan. Co-director Joe Russo explained:
He's from a planet called Titan. Many years prior to the film his planet was experiencing a cataclysmic shift. They were running out of resources, and they were overpopulated. Thanos made a recommendation that they exterminate half the population randomly in order to save the rest of the population. Of course, the Titans rejected his notion, branded him a mad man, exiled him, and the planet ended up dying. So he has taken it upon himself to go planet-by-planet throughout the universe and wipe out half of the population of each planet as a way to correct the planet and bring it back into balance.
Marvel Studios president Kevin Feige briefly touched on Thanos' backstory when it was confirmed that we would see Titan in Avengers: Infinity War, but Joe Russo provided far more detail when he and his brother, Anthony Russo, were speaking with The Telegraph. We'd already learned that Thanos had failed to save Titan, and it turns out the reason he avoided the same fate as the rest of his kind is because he was banished for wanting to kill off half the population to save the other half. Talk about taking 'the ends justify the means' to a whole other level. With his people gone, now Thanos has taken it upon himself to enact this same plan on other worlds, which explains why he's a feared warlord across the galaxy. Since the Russos have already said we won't see Thanos' backstory in Infinity War, ideally all this information will be passed along through dialogue/exposition.
That brings us to the Infinity Stones, which are powerful enough on their own, but when all six are wielded together, the user is given god-like abilities. Joe Russo continued:
Once he hears about the stones, he realises that if he can get hold of all of the Infinity Stones he can control the entire universe. With the snap of his fingers he could correct the universe permanently by removing 50 per cent of life from it. So, these are incredible stakes for our characters. If the Avengers were built for anything, it's for stopping Thanos from completing his goal.
Those familiar with Marvel history know that Thanos' motivations in Avengers: Avengers: Infinity War are in sharp contrast with his comic book counterpart's. In the Marvel Comics universe, Thanos is in love with the personification of Death, and he killed millions and collected the Infinity Gems just to impress her. While there was speculation for a time that Thor: Ragnarok's Hela might fill Death's place in the MCU, and Josh Brolin's Thanos would try to win her heart, that didn't come to pass. Nevertheless, Thanos still wants to commit genocide on a universal scale, and it will be up to the Avengers, Guardians of the Galaxy and other select superheroes to stop him.
We'll see if Thanos is successful when Avengers: Avengers: Infinity War is released in theaters on April 27. To learn what other MCU projects are in development, head to our Marvel movies guide.