With Avengers: Age of Ultron coming out next summer, and Marvel's phase 2 coming to a closing, there is a huge amount of fan speculation surrounding the titular villain's origin. I am using information from every Phase 2 movie up to this point to draw all possible conclusions, so to those who have yet to see The Winter Soldier, Thor: The Dark World, and Iron Man 3 (if you haven't seen that one by now, shame on you!) MAJOR Spoiler Alert!!!!!!!!
Here are the possible creators:
He was the original creator of Ultron in the comics, and the Ant Man movie (Pym's alter ego) is set to release almost immediately after Avengers 2. Though they deny Pym's presence in Avengers 2, there is a wonderful invention called lying.
Tony Stark via JARVIS
The teaser trailer first seen at Comic-Con last summer can now be found easily online. It show's a damaged Iron Man helmet transform into the head of Ultron. This could be a huge hint that Ultron is created from one of Tony Stark's armored suits. In Iron Man 3, there was a certain scene in which JARVIS (the A.I. that runs Tony Stark's.... well.... everything!) appeared to be malfunctioning. "... I seem to do quite well for a stretch and then at the end of the sentence I say the wrong cranberry." The scene may have just been for comic relief, but it would be just terrible if the malfunction was serious, and the A.I. that controls Tony's house, his machines, and particularly his suits, went bad. But since Paul Bettany (the voice of JARVIS) has been cast as Vision in Avengers: AoU, with James Spader cast as Ultron, that's looking a little unlikely. But Ultron could still come from an Iron Man suit, but perhaps not one of Stark's, which brings us to the next possibility....
AIM/Hydra via The Iron Patriot/Arnim Zola Android Body and Arnim Zola's A.I. program/Loki's scepter
Now this one is a little more complicated, considering it's multiple theories mushed into my own evil Ultron theory casserole!
In the conclusion of Iron Man 3, the Iron Patriot armor was the only one remaining, after Tony Stark retired from his duties as Iron Man and destroyed all of his suits. The Iron Patriot armor was previously known as War Machine until it was upgraded and repainted by A.I.M. (the bad guys), who later hijacked it and controlled it later from a long distance and used it to attack important government people. And that wasn't the first time. Whiplash did pretty much the same thing in Iron Man 2. In fact, it's becoming a recurring gag. Rhodey just keeps losing that darn suit. But now, since it's been revealed that A.I.M. is allied with HYDRA, it could pose quite a threat if they hijacked it again. But then again, HYDRA may already have the the parts they need for their own killer robot: Arnim Zola's blueprints.
Remember the doctor from Captain America: The First Avenger who worked for Hydra and was always bullied by the Red Skull? Well, after the movie, he joined up with SHIELD, still loyal to HYDRA, found out that he was dying, uploaded his brain into a computer, developed a program to detect and eliminate threats to HYDRA, called "Project Insight" and created blueprints to and android body!
It seriously looks like it could potentially be how Ultron is created- only when taken out of context. It is more than likely a red herring since Arnim Zola did the same thing in the comics and this stupid thing was the result..
But than again, Project Insight could still be important to Ultron's creation. But Joss Whedon said "He's not a creature of logic - he's a robot who's genuinely disturbed. We're finding out what makes him menacing and at the same time endearing and funny and strange and unexpected, and everything a robot never is." That does NOT sound like any kind of A.I.-gone-bad scenario. That usually starts with them deciding that "humanity is a threat to itself". But that is too logical and impersonal. That leaves one other thing:
Let's go back to The Avengers. Loki carried around a blue glowing scepter that he used to mind control people. Many people assumed that it's power came from the tesseract. At the end of The Avengers, Thanos was revealed to be the guy in charge, seeking the tesseract, giving Loki the scepter, sending an army, etc. After that, in Thor: The Dark World, it was revealed that the tesseract, along with the Aether was one of six "infinity stones" (clearly a reference to the infinity gauntlet). And while the tesseract is all the way in Asgard, the scepter is on Earth, fully functional, and in the hands of HYDRA, meaning it could very well be the movie equivalent of the mind gem. I think it will give life to Ultron. Here's my explanation:
The Aether is red like the power gem and gives Malekith power
The tesseract was a doorway like the space gem and transported Loki to Earth
And yet was used as an unlimited power source
so, theoretically, if the scepter is the mind gem, it should be a power source no different than the tesseract. Oh, and HYDRA has weapons like the ones powered by the tesseract.
And their allies have a suit meant to be controlled from a distance.
Perhaps, they tried to use the scepter as a power source for the suit, but it instead makes the suit become sentient. It's a little far-fetched, but it seems to be the most likely out of the alternatives. Share your thoughts in the comments below.