Avengers: Age of Ultron has opened to $191.3 million from 4,276 theaters in North America alone; suffice it to say, a lot of people have seen the movie. And, so far, no one that I've seen has mentioned what struck me as the most important aspect of the movie: The enemy isn't Ultron. It's Tony Stark.
At the beginning of Age of Ultron, the Avengers have Loki's scepter firmly in hand, but Stark convinces Thor to spend three more days on Earth, presumably for a party. It was a lie. In reality, he wants to get a closer look at the technology within the scepter. As it happens, it contains an alien AI.
Stark convinces fellow Avenger Dr. Bruce Banner to help him study the technology as well as to keep their work on the down-low. Why would Stark shut the rest of the Avengers out of this very important plan? Because he didn't want the Avengers to stop him. Tony doesn't think about what it means to be part of a team. (Also, in the real world, scientists who work with unknown substances spend time studying the material, and they take precautions. Knowing he only had Loki's staff for three days, Stark barreled ahead -- screw safety measures when working with an alien technology. That's some Prometheus "Take our helmets off on the alien planet"-level forethought, right there.)
In short work, Ultron is born.
Although it's not explained in the movie, it's clear that Ultron is based in part on Stark. 1) The original Ultron, who first appeared in The Avengers comic in 1968, was the creation of Hank "Ant-Man" Pym, based on Pym's brain patterns. 2) Tony refers to him as "Junior." 3) His dialogue is pure Stark even to the point at which other characters remark upon it.
In an extreme, but logical, conclusion of Stark's self-serving personality, Ultron becomes a murderbot. Ultron's first act is to kill Jarvis, and his second and last (attempted) acts are to kill the Avengers and all of humanity. A robot based on, say, Captain America's personality would have no compulsion to kill. What does it say about Stark that Ultron does?
But it's not that he refused to feel an ounce of remorse when his own creation started to murder up a city that makes me see Stark as the villain of this story. It's his reaction when confronted by Thor.
Thor makes an extremely valid point that "this could have been avoided if you hadn't played with something you don't understand." But Stark doesn't respond to the accusation and accept blame. Instead, he turns the talking point away from any potential criticism into a justification as to why humanity needs Ultron: to defend us from a hostile alien army, like the Chitauri.
He had a chance to say, "I'm sorry." He didn't take it.
To sum it up, he lies to create a planetary defense system (which would benefit Stark Industries immensely); doesn't tell his teammates about his plans to eliminate their jobs; creates an AI based on his own personality that murders people; and doesn't apologize for it. Forget Thanos: It's just a question of time before Stark becomes the villain of the Marvel Universe.
Here's hoping that the cities of Johannesburg and Guateng, where the Hulk-buster scene was filmed, sues Stark for damages caused by his Iron Man suits.