It's been pretty fascinating to see how the different movies approach big questions of culture and politics, which are always extremely controversial grounds but especially today. There's often a lot of discussion in the world of movies about the "death of monoculture." Meaning that none of us are watching the same shows, the same movies, or living in the same spheres. We are an increasingly diverse society and culture with everyone tucked away in different bubbles.
The Star Wars movie franchise wants to be as close to "mono-culture" as they can, building off the uber-popular original trilogy and the place it holds in American culture. However they also want to be progressive in values so they don't really pull off the feat of creating a universal myth that speaks to what American film-goers believe about culture and power. They're also making crappy films and it's not obvious if that's due to the attempt to adhere to progressive doctrines, flaws in the original Star Wars universe that Lucas created, a massive failure in execution, or some combination of everything I've mentioned above.
But the Marvel Universe? Universal acclaim and appreciation, improvement on the original universe (the comic books), and stunning execution. Watching the Marvel studios weave the same characters in and out of each others' stories from movie to movie is amazing. They're accomplishing something here that's much grander than anything Lucas or his successors of the Star Wars franchise have done.
Infinity War was one of their most impressive achievements to date, in part because of all the characters they effectively utilized in the story and in part because Thanos proved to be the ultimate foil to the Avengers' building mythos on culture and power.
Each sub-section of the Avengers' greater franchise tends to center around a different theme but they all tend to exist within the greater theme of self-actualization.
It all started with Iron-Man, who's movies and character always center around guilt and the need to be sanctified. Tony Stark became powerful because he was a genius who inherited both genius and wealth from a worthier father. He misused his power early on and then had his "come to Jesus" moment when trapped in a cave under guard from terrorists and realized that his haphazard approach to wielding his genius and overseeing his business had led to tremendous harm around the world.
His trilogy centers around cleaning up that leftover mess while his post-Iron Man 3 character is more concerned with trying to handle the weight of responsibility he feels for the world and what he's had to do to defend it as Iron-Man. His self-doubt as a hero, shielded off with witty banter, has driven the franchise to this point.
Captain America has always been the model for the determination of "who gets to wield power?" He's chosen for the super soldier program in WWII because he's identified as a person of such great character and sacrificial love that he can be trusted to wield superpowers on behalf of others.
His first movie is about him acquiring those powers and earning the trust to put them to use, his later films in the trilogy center around the theme of trust in "the right people" over weapons or institutions. He's loyal to his friend Bucky but is faced with challenges over who to put trust in and time and again he choses to trust people based on character.
Thor's storyline centers around his self-actualization as "the good king" archetype. In each of his movies he becomes a better and better potential king of Asgard and his power grows with his expanding responsibilities. His story arc doesn't really coincide much with the Avengers' movies like it does with Capt America or Tony Stark. His growth usually comes in his own movies and then he puts whatever he's learned to use in the following Avengers' movie. At least up until "Infinity War" when he carries a larger role, although even here he's more just assuming a mantle that he already earned in "Ragnarok."
The other original Avengers see their characters developed entirely in supporting roles. Then there's Spider-Man who serves two major roles in the franchise. The first is as a young man who has to learn that the power and responsibility are outflows of internal strength and can't fuel internal strength. The other is to be a son figure for Iron-Man that drives Stark's growth from a responsible adult into someone that is learning to protect and expand his responsibility to the next generation. They both learn from each other and their dynamic has been one of the most successful things that the movies have accomplished. No doubt that's why Spider-Man's disintegration in "Infinity War" is the one that gets the most camera time. The self-doubt this will create in Starks undoubtedly serves as a major theme in the next feature.
The Guardians of the Galaxy have their own thing going on that is rich and fascinating. The themes of their movies tend to be less around the individual and more the collective society. In the first movie Star-Lord and his friends learn how to grow beyond themselves and into a role of protecting society, stunningly from a villain that is a rather obvious stand-in for Islamist terrorism (religious zealot who sends suicide bombers?).
In the second movie there are all sorts of wonderful rebukes of modern society's narcissistic self-absorption in the social media/internet age. The big juxtaposition on film is between the nitty gritty of building and maintaining relationships between real people (particularly family) and then the golden folks that turn war into a video game in which they are safe from any real consequences and a super-villain that wants to get rid of anything in the universe that isn't his own self.
Dr Strange is just another super power guy who has an amazing knack for acquiring power (first as a star surgeon and then as a master of the mystic arts) but has to learn A) it's about serving others and not himself but also B) that this "true greatness" means risking failure in a way that he's always shied from. When it was all about himself, failure was untenable, when it was about others then it could no longer be about protecting himself. That last nuance makes that movie and saves it from being an unneeded hero story with a mystical, multi-verse backdrop.
Altogether, the Avengers join forces to become the team that gets you when you try to wield power without heeding the lessons about sacrifice, prudence, and responsibility that the Avengers all learned in their respective fiefdoms. Break that code and it will be avenged.
In the first film Loki, apparently acting on behalf of Thanos, is aiming to take over the world and hand over power to Thanos in the form of the infinity stones that are on Earth in exchange for the rule and vengeance he seeks over his brother. The story arc is driven by the Avenger's struggle to actually coalesce and get on the same page so that they can get the job done.
The second film features Ultron, the product of Tony's first attempt to sire the kind of "son" that can take over the job of protecting the Earth. It goes poorly, which creates new guilt and self-doubt for Iron-Man to work through, and the Avengers again have to work through some stuff before being avenged on the power-mad Ultron. The moral of this story is mostly to be found in the growth that comes through failure, Stark spends the following two films (Capt. America "Civil War" and "Spider-Man: Homecoming") trying to figure out how to actually raise up permanent solutions for the defense of the Earth. Even the creation of Vision didn't quite succeed there and the greater hope is found in his fathering of "Spider-Man."
Finally, "Civil War" breaks up the Avengers because Starks' self doubt and attempts to deal with it don't jive with the Captain's own confidence and conviction. They both reach a sort of middle ground at the end of the movie even though they battle each other and break up. Starks learns from Spider-Man that he can't abdicate his responsibility to wield power to someone else just because of lack of self confidence while the Captain learns that he's not infallible or incapable of making mistakes. The balance between those two perspectives meet the ultimate challenge when facing Thanos.
Thanos is basically exactly what you'd be afraid of the Avengers becoming, an exceptionally powerful being who has the wrong idea about why to wield that power and absolutely zero self doubt when doing so.
He is also perfectly designed to evoke fear of totalitarian rule from whichever end of the political spectrum you tend to oppose. I came out of the theater thinking that Thanos reminded me of 20th century communist rulers who'd wiped out millions of people trying to remake their societies. Then I read a good review by a liberal critic who found that the ending reminded him of the devastation he and his friends had felt after Trump won the 2016 election. I thought that was kinda silly but it just points to the way in which Thanos' "will to power" philosophy and approach was well calibrated in this movie to be opposite universally held precepts in the culture rather than trying to solely represent one side or the other (a flaw in "the Last Jedi").
That said, there is a sort of conservatism or prudence in what the Avengers tend to be avenging. Namely, they are always working through the world as it is and trying to maintain a slow and steady order, protecting people and allowing things to go along at a natural and easy going pace. They never use their powers as a way to remake society but always to protect it and allow it to go along at its own course without disorder and chaos ruling the day.
Thanos wants to enact drastic change, to rip of the band-aid so to speak. The really revealing expose of what Thanos embodies is best captured by this exchange with his "daughter" Gamora:
Thanos: Little one, it's a simple calculus. This universe is finite, its resources, finite. If life is left unchecked, life will cease to exist. It needs correcting.
Gamora: You don't know that!
Thanos: I'm the only one who knows that. At least, I'm the only one with the will to act on it.
For Thanos it's all pretty simple and straightforward, as it always is for ideologues. Political ideology always simplifies everything into really easy black and white issues and then it's simply a matter of whether you're willing to go far enough to win and enact your program on society. To Thanos, the main problem in society is that no one will have the moral courage to be strong enough to enact the right ideology for the good of everyone else.
The Avengers, who are always wrestling with the limitations of their power and ability to see what's best, have to step in to stop Thanos from wiping out half the universe. It's assumed by the filmmaker that his view of life as a complex system that should have a "handle with care" warning sticker attached is a failure. You are clued in to this by the fact that he regularly enacts a price and a toll on others with torture and murder whereas the Avengers choose sacrifice. The one exception is when he gives up Gamora, which seems to basically amount to filicide and the juxtaposition between that and Starks losing Spider-Man is hopefully something that will be explored in the next movie.
These movies are mostly kind of pagan with their depictions of the universe, it's formation, and the powers that govern it. However, there's still a Christian worldview behind it despite America and Hollywood's increasingly post-Christian culture. You can see it in the emphasis on personal responsibility, the power of sacrifice, and the importance of humility. Ideology is the most grave threat to Western civilization and Thanos embodies it against the more measured and careful approach worked out by our Marvel heroes.
But you can be confident it'll all end alright in the end after this next film...