Ultron is a different kind of super villain. He tends to be unrelenting and smarter than most of the heroes he faces. With access to almost every computer database this robotic evil tends to know every power and weakness a superhero may have. This is why having him as the next villain in Avengers 2 sounds like a great idea. But how do you portray a robot gone off his programming? Well you cast James Spader of course.
Known more for his comedic roles in the past handful of years, like his appearances on The Office or his role on Boston Legal, Spader has always done a good job as coming across as cold and calculating. Hearing his voice calculate out Iron Man's demise could be just the right fit for the character. I do wonder if they'll make Spader where a full outfit, helmet too, when playing the character or if like usual the actor will want his face shown.