And this is Tony Stark fault again. I mean seriously, this, underestimating AIM, Ultron, what's he going to do to Banner/Hulk, Civil War. It's like Tony Stark is one walking bad decision maker.
I think that Ultron is the AI of Tony Stark's old lab. After his house got demolished and he decided to give up on the Iron Man thing, the main AI of his lab didn't agree with that. Then it went rogue, went insane, uploaded itself into an Iron Man body, started replicating itself, and became Ultron.
Actually Tony doesn't give up on him being IronMan, just "cutting it down" a bit. Also some speculated that since SHIELD was in a mess, Tony made an army of robots to help the Avengers to round up HYDRA operatives and secret bases. The problem is the AI was the robot assistance (not JARVIS, the other robot assistance that got destroyed but salvaged during the ending of IM3) and it probably start to evolve more and more and reaching singularity, thus creating ULTRON. Since it consider humanity as the root of all problems it decided that ruling them is better because AI is more systematic and structured.
Oh the Hulk Buster? It's not posessed by Ultron. In the book it's actually part of the "contingency plan" when Hulk go out of control. Stark build that thing for one purpose only: To stop Hulk. In the book Stark ended up launching Hulk to space and he landed on another planet. Some speculate that this is when Guardian of the Galaxy will meet the Avengers.