I just found an article called African Americans and Serial Killing in the Media The Myth and the Reality, By Anthony Walsh, Homicide Studies, November 2005[PDF].The subheading says
There were many expressions of shock and surprise voiced in the media in 2002 when the Sniper” turned out to be two Black males. Two of the stereotypes surrounding serial killers are that they are almost always White males and that African American males are barely represented in their ranks. In a sample of 413 serial killers operating in the United States from 1945 to mid-2004, it was found that 90 were African American. Relative to the African American proportion of the population across that time period, African Americans were overrepresented in the ranks of serial killers by a factor of about 2. Possible reasons why so few African American serial killers are known to the public are explored.
He starts with this amazing popular culture moment:
In Copycat (Fiedler, Nathanson, & Amiel, 1995), Sigourney Weaver plays a criminal psychologist and expert on serial killing. In the opening scene, she is giving a lecture in an auditorium in which she asks all the males in the audience to stand, emphasizing that serial killing is primarily a male behavior. She then asks all African American and Asian American males to sit down, leaving only White males standing as representative of serial killers. The message that viewers of Copycat get is that only White males commit these heinous crimes and that members of other races or ethnicities never do.
You are invited to imagine what would happen to a professor who did it the other way around.
This stereotype is pervasive in the United States, so much so that the only African American serial killer that any of my criminal justice department colleagues could name was Wayne Williams (the Atlanta child killer), and the only mass or spree killer named prior to the 2002 John Mohammed and Lee Malvo sniper spree was Colin Ferguson, the Long Island train mass killer. A Bowie State University graduate thesis available online explicitly states that “there is an absence of African American murderers within the realm of serial homicide investigation”(Duncan, n.d., p.1). This statement apparently passed muster without comment by a thesis committee. A commentator in the Harlem Times expressed shock and disbelief when the D.C. Sniper turned out to be two Blacks, because “white guys have pretty much cornered the market on mass murders and serial killing”(Charles, 2002). Psychologist Na`im Akbar stated, “This is not typical conduct for us. I mean Black folks do some crazy stuff, but we don`t do anonymous violence. That`s not in our history. We just don`t do that”(cited in White, Willis,& Smith, 2002, p. 2). Given this widespread “authoritative” opinion, it is no wonder that a columnist in the Black Commentator wrote, `There aren`t any black serial killers.`
It is one of the mysteries of modern criminology that a group responsible for a highly disproportionate number of homicides of all other types has gained a reputation for not producing serial killers, or at least for producing a disproportionately low number of them.
Read the whole thing: [PDF].