Generally, the characters come first. Okay, not usually the main characters, but a character. A face, a name, a quirk, a history. So why have you given me this whole story and no one to populate it save the hollow plastic redshirts?
I at least need the main two.
Let's call the antagonist Outrage, shall we? He is my mysterious, deliberate, ruthless, cruel murderer who tries to express his frustration with all the wrongs of the world by attacking its symptoms and symbols. He is bitter and violent, long past the breaking point of a rope stretched thin by too much empathy with the suffering of the many. So much that he's discarded the importance of the suffering of the one, even his own. Is he a former reporter, maybe, jaded with the shallowness of the public eye? Or maybe he spent too long mewed up with only the news for interaction with the outside world. Prison, perhaps, or a hospital bed. Maybe he's a soldier sent home injured and forgotten in a hospital bed somewhere especially neglectful. He's definitely American, ethnocentric even as he rages against ethnocentricity. Blindly hypcritical. Probably of middle-class upbringing with a tense but not an abusive youth. Fails to see that media coverage is the fault of the media, not its subjects. Possibly once tried to bomb a televison station or newspaper office and had it go entirely unnoticed.
I think, when he realizes that media coverage of his own exploits is doing precisely the thing he hates so much, he will do one of three things: Simply stop, turn himself in, or become his own last victim.
And then I also need Investigato, my methodical, level-headed, compassionate protagonist (Sounds boring, doesn't he?). He will have to grasp Outrage's patterns and motivations without ever sinking to agreeing with him, even if he might share some of the same views of media injustice. Empathy for Outrage's reactions, but not his actions.
I have a little bit of the shape of him. I think he is a married man, with an adopted son or daughter. He is, by necessity, an FBI agent. What drove him there, with his background in journalism and psychology, and his happy childhood, untouched personally by crime?
Think on it.