Ultron is convinced that the only way to bring peace and order to the world was to destroy humanity.
Rumor says that Iron Man’s artificial intelligence computer system Jarvis turns against Iron Man (Tony Stark).
JARVIS becomes evil and will be on the side of Ultron to help create its suit and mask from the Stark weapon blueprints.
“Hope is a human delusion”
a classic quote from Ultron
Another storyline is that Ant-Man is the one who creates the robot that is called Ultron and he turns on him. But I think Marvel won’t bring Ant-Man into Avengers 2.
Most likely that JARVIS gets infected by the “Ultron-Virus” (perhaps from Thanos?) and Stark Industries built the evil robot! Later Ultron will modify and upgrade himself and takes over trying to destroy the Avengers.
Check out the Iron-man suit morphing into Ultron’s mask – an extract from the Avengers 2 – Age of Ultron Trailer.