Tony Stark's vision of the future from Age of Ultron pretty much comes true at the end of Infinity War. His worst fears have come to haunt him in some pretty incredible connective tissue from the Marvel Cinematic Universe. In case it wasn't obvious, there are major SPOILERS for Infinity War ahead, so read at your own risk. The Russo Brothers have been up front from the start that the movie was going to see the deaths of some superheroes and that the story was going to be told through the perspective of Thanos. As it turns out, they were not kidding around.
*** SPOILERS START NOW ***
During the battle on Titan, Doctor Strange ends up giving the Time Stone to Thanos in exchange for Tony Stark's life. Later, and before Strange dies, Stark asks him why. Doctor Strange reveals that he had to do it before he turns to dust, which makes sense because he's the only one who has seen the future to see all of the possible outcomes. Out of the millions of outcomes that he mentioned, there was only one that ended up in the defeat of Thanos. Doctor Strange knew exactly what was going to happen and how to make it happen, leaving Tony Stark and Nebula alone on Titan.
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Thanos is able to obtain all of the Infinity Stones by the end of Infinity War and makes good on his promise to snap his fingers. He ends up killing half the universe to save the rest of the universe, which was his plan all along. His genocide ideas were thought to be correct because of what he did on Gamora's home planet, killing everybody off because of the poverty and starving. The Mad Titan believes that he rescued Gamora, saving her life and giving her a new purpose. Thanos reveals that her planet is now thriving after he wiped out the population, leading to the mass killings of half of the universe in Infinity War.
But at what price? Thanos achieves his goal after being attacked by Thor and then is magically transported and all healed from his wounds on a distant planet. However, before that, he has a talk with a young Gamora where she asks what his deed cost him, to which he responds, "everything." Thanos had to sacrifice the only thing he loved, Gamora, to obtain the Soul Stone on Vormir. Thanos is last seen on another planet where everything seems to be perfect, like he had imagined, but he still doesn't seem satisfied.
Infinity War only has one post-credit scene and it comes at the very end of the movie. The scene features Nick Fury and Maria Hill back on Earth as the power of Thanos' snap becomes real. Hill disappears, and Fury goes into code red, picking up a pager-like device to contact Captain Marvel before he vanishes. Captain Marvel will be featured in Avengers 4, and it looks like there will be some time travel elements used to see the future through that Doctor Strange envisioned from the beginning.