The Real Hero of Infinity War
The hero is Thanos.
Infinity War is a modern epic poem. The classic hero faces a journey fraught with many obstacles, each of which he must overcome during a long journey. Only once the final obstacle has been beaten, and the journey completed, does he at last complete the task he set out to complete.
Thanos, for better or worse, wants to end half the lives in the universe. He's been fixated on his dream for his entire adult life. As an aside, some ask why not bring balance to the universe through some other means, but that's a question that ignores human (titane?) nature. When Thanos acquired the power to do as he long desired, he did what most megalomaniacs would do in the situation-- he went with what he'd always believed is the right thing to do. People don't change just because they get rich or powerful.
In any event, Thanos wants to destroy half of all life. To do so, he must embark on a quest to gather the six infinity stones, and to acquire each, he must overcome an obstacle, make a sacrifice, defeat a foe. Along he way, he journeys through the vast universe. It's the classic epic, and just because we the audience consider the Avengers to be "our" heroes does not change the fact that only Thanos fits perfectly in the role of the poem/film's hero.
Thanos even fits the role of cinematic hero. Consider the ending-- just as Thanos is about to claim the final infinity stone, he's seemingly defeated when the Scarlet Witch destroys said stone. But he's not. He overcomes that with the time stone. And when Thor seems to have him, he again wins at the last possible second. That's normally what the hero does in a film-- appears to be defeated, then wins through some unexpected act of heroism.
It's a wonderful story, and as far as I know, an unprecedented one in the annals of comic book films. Infinity War is the epic tale of Thanos, and his quest to bring balance to the universe.