Well, Slate fills us in on the details:
Why can't newspapers drop all these modifiers and just go with killer?
They can, but they usually don't out of deference to the judicial system. In a criminal case, of course, the defendant is presumed innocent, and prejudgment in the media could bias the jury pool. Any publication that chose to identify Hasan as the killer would be immune to libel claims, since the statement would clearly have been made in good faith. But reporters traditionally hedge until the legal system delivers its final verdict. Some news stories attribute conclusions to public officials so they don't seem like statements of fact. ("Prosecutors claim" can be a handy phrase.) The Associated Press Stylebook goes so far as to recommend "John Jones, accused of the slaying" over "accused slayer John Jones," because the latter more strongly suggests prejudgment.
The more the phrase sounds like legalese, the more dangerous it becomes for the news outlet. So you'll often see people described as pirates even before trial, but an arsonist would almost always be "suspected" or "alleged" until his trial was over.So there you go.