After seeing the awesome Super Bowl trailer for the upcoming Captain America 2: The Winter Soldier, I would like to somewhat amend my theory on Ultron's creation.
It has been stated that CA2 will have a direct connection to Age of Ultron. Considering how Ultron was (originally) born from Hank Pym's brain pattern, it would be plausible that he would have a similar birth in the cinematic universe.
Early theories have stated that Tony Stark will be the inadvertent creator of Ultron. Which would make sense given the universe that has been set in the movies. If Ultron is based on his character, that would explain the egotistical behavior that Ultron usually exhibits. But Tony Stark has no particular hatred toward his fellow man, in fact he has shown throughout the Iron Man series to care more and more about humanity. So where does that hatred come from?
In the new trailer we see the Winter Soldier hooked up to a computer for any number of reasons (analyzing condition, implanting directions, etc). He is a cold calculating programmed cyborg. But when he realizes what he's been doing (beyond his control) he harbors guilt and loathing. If the A.I. picks up on Bucky's raw emotion, guilt can turn toward resentment, which can turn toward hate. And who better to direct this hate toward than all of mankind?