Lee Harvey Oswald, Mark David Chapman, John Wilkes Booth, James Earl Ray. None of these names are seemingly ever said without their middle name. It's hard to think of non-assasins who are referred to this way. Middle initial sometimes but really rarely full name including initials. Why is this the case?
The short answer is that when the FBI releases a suspect’s name, they release the full name, and that’s what the press runs with, thus how it’s delivered to the masses via the news and subsequently engrained into the cultural/societal lexicon.
It’s not just assassins; think of all the serial killers in federal custody known by their full name.