WARNING: This post contains SPOILERS for Avengers: Infinity War.
As the central antagonist of Avengers: Infinity War, Thanos is the greatest threat the Avengers have ever faced and the greatest villain the Marvel Cinematic Universe has yet known. First teased in the post-credits scene for 2012's The Avengers, the build-up to seeing the Mad Titan clash with Earth's Mightiest Heroes has been a slow one.
Actual appearances otherwise have been scarce, emotive discussions of his cruel, unflinching demeanor in the Guardians of the Galaxy films being his biggest advocate, while the mystery of the Infinity Stones has gradually brought he and Stark and co. closer together. And now that he's here, it's plain to see why Gamora and Nebula were so trepidatious about anyone being in his way.
Despite not having the best track record, the MCU has created some great baddies. The violent insurgent Killmonger in Black Panther and the hard-done-by criminal everyman Vulture in Spider-Man: Homecoming were both compelling foils for their protagonists. We've seen cosmic-scale enemies before in Ego from Guardians Vol .2 and Dormammu from Doctor Strange, and we've seen pathological dedication to a revolutionary ideal in Red Skull and Ultron. But none have come with the mix of ferocity and grandeur that Thanos brings – none have been as simultaneously capable of and ruthlessly dedicated to affecting the entire universe as the last living Titan, a fact that makes watching the Avengers square up to him all the more exciting and distressing.
Thanos Doesn't Care About The MCU's Heroes
Gathering the Infinity Stones isn't something Thanos takes lightly, bringing with him as he does his Black Order. But when it comes to the opposition of Marvel's heroes, they're an inconvenience for him, a nuisance. Thanos is a professional eradicator. He's a galactic tyrant whose major skills include taking a planet by force, picking its best children for personal training and then having half the remaining population killed. To him, having to fight some enhanced humans is the equivalent of some extra paperwork that can be doled out to your subordinates because you're the one in charge, hence sending his children to Earth for the stones there while he retrieves the others and deals with his precious Gamora.
The 'Infinity War', insofar as dozens of heroes come together to defend the universe, doesn't matter to Thanos. To us, we've spent movie after movie with these characters, watching them grow and evolve in this massive inter-connected franchise built on decades of comic book history. Their journeys to this point have meant a lot to a lot of people, something reflected in the recurring use of self-aware dialog and the growing number of heroes and villains inspired by in-universe events. Each story of the MCU is built on the reverberations of every other story, creating a massive, interwoven pop-culture machine. Avengers: Infinity War is the first in the two-part apex of this decade-long, 20-something movie build, with this one giant face-off being the endgame of many of the story threads spawned in the interim (and tidying up some inconsistencies). But none of that matters one iota to the Purple One on his quest to restore order to the universe.
Almost all the villains we've previously seen approached the protagonist with a ferociousness that reciprocates the reverence the audience is meant to have for them. They don't under-estimate the hero, some of them dedicating their lives to getting the chance at a one-on-one fight. In the opening scene of Infinity War, Thanos beats up the Hulk in what one of his children calls “a bit of fun.” The strongest Avenger, the single most entertaining part of the whirlwind third act in The Avengers, is outdone in a fist fight at the beginning of Infinity War without the despicable Titan even breaking a sweat. Thanos is fearsome because when he's on-screen, the Marvel Cinematic Universe cares more about him than the heroes.
We believe the amazing Spider-Man and the master of the mystic arts Doctor Strange can win because we've been trained to do so, but we also know what kind of monster Thanos is. They'll win because that's what they do, right? Iron Man and Captain America have figured out how to beat everyone else, surely this won't be any different, right? Watching Thanos go from nigh unstoppable to completely unstoppable very quickly tells us they really might not come out alive this time. The Mad Titan's repudiation of the MCU's narrative worship of its heroes creates a deep uncertainty in our expectation that follows through each encounter toward the inevitable, horrifying conclusion.