The Avengers are the titular characters of Avengers: Infinity War, but the movie's actual star is Thanos according to directors Anthony and Joe Russo. In an interview with The Telegraph India, the two brothers said that it is "without question" a Thanos movie - and also reveal ithat the movie is told from the point of view of the Mad Titan.
"We always try to bring in topical themes into these films because they function on such a wide scale that we really want the audience to feel a sense of immediacy in their own lives about what they’re watching," said Joe Russo. "Every villain is a hero in his own story and believes that what they’re doing is right. They’re just in conflict with the rest of the world. Thanos happens to believe that what he is doing is right, and he behaves nobly towards that goal. But he will not stop until he achieves the goal because he believes that there is weakness that stands between him and the completion of the goal."
"We thought it was fascinating to tell a story from the point of view of a villain," he continued. "So when you watch the film, you’ll see that the film is told from Thanos’ perspective. That offers a unique insight into our heroes, but it also offers a unique insight into villains and how they think."
As for insight into Thanos himself, you don't have to wait until the movie to understand him - Russo spells it out in this interview.
"Thanos is this virtually indestructible character who’s stronger than the Hulk, and has invincible skin. 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," Joe Russo explained. "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."
"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% of life from it," Russo continued. "So, these are incredible stakes for our characters. If the Avengers were built for anything, it’s for stopping Thanos from completing his goal."