From The Boston Globe:

[Brian] Williams is among a small cadre of scholars from across the world pushing the rather contentious idea that some suicide bombers may in fact be suicidal. At the forefront is the University of Alabama's Adam Lankford, who recently published an analysis of suicide terrorism in the journal Aggression and Violent Behavior. Lankford cites Israeli scholars who interviewed would-be Palestinian suicide bombers. These scholars found that 40 percent of the terrorists showed suicidal tendencies; 13 percent had made previous suicide attempts, unrelated to terrorism. Lankford finds Palestinian and Chechen terrorists who are financially insolvent, recently divorced, or in debilitating health in the months prior to their attacks. A 9/11 hijacker, in his final note to his wife, describing how ashamed he is to have never lived up to her expectations. Terrorist recruiters admitting they look for the "sad guys" for martyrdom.

Not everyone is swayed by his argument. Mia Bloom is a fellow at the International Center for the Study of Terrorism at Penn State University and the author of the book, "Dying to Kill: The Allure of Suicide Terror." "I would be hesitant to agree with Mr. Lankford," she said. "You don't want to conflate the Western ideas of suicide with something that is, in the Middle East, a religious ceremony." For her, "being a little bit wistful" during a martyrdom video is not an otherwise hidden window into a bomber's mind. Besides, most suicide bombers "are almost euphoric" in their videos, she said. "Because they know that before the first drop of blood hits the ground, they're going to be with Allah." (Lankford counters that euphoria, moments before one's death, can also be a symptom of the suicidal person.) 

 One study in the academic literature directly refutes Lankford's claim, and that's the University of Nottingham's Ellen Townsend's "Suicide Terrorists: Are They Suicidal?" published in the journal Suicide and Life Threatening Behavior in 2007. (The answer is a resounding "no.")