Yes. (Not a very surprising conclusion, is it?) Recent research shows this to be true for Iraq War veterans:
[A study led by Shira Maguen of the San Francisco VA Medical Center and University of California-San Francisco] examined the mental health impact of reported direct and indirect killing among 2,797 U.S. soldiers returning from Operation Iraqi Freedom. Data were collected as part of a postdeployment screening program at a large Army medical facility. Overall, 40% of soldiers reported killing or being responsible for killing during their deployment. Even after controlling for combat exposure, killing was a significant predictor of posttraumatic disorder (PTSD) symptoms, alcohol abuse, anger, and relationship problems. Military personnel returning from modern deployments are at risk of adverse mental health conditions and related psychosocial functioning related to killing in war. Mental health assessment and treatment should address reactions to killing to optimize readjustment following deployment. ...
Military personnel who have killed may experience significant shame and/or guilt and need to know that they will be allowed to explore the impact of killing in a safe and supportive environment (e.g., Veterans Affairs). They also may have received criticism or been subject to insensitive questioning by acquaintances, friends, or family members that cause them to be weary of speaking to others about this sensitive issue, especially when they fear others will not understand or judge them for their actions.
Source: "The impact of reported direct and indirect killing on mental health symptoms in Iraq war veterans," Journal of Traumatic Stress, Volume 23 Issue 1, Pages 86 - 90
An earlier study by the same research team shows a similar correlation between killing in combat and increased incidence of PTSD in the Vietnam War veteran population.
In educational interest, article(s) quoted from extensively.
On the Iraq study, Tom Vanden, USA Today:
Soldiers such as Grant Speakes, who say they killed enemy troops in combat, are at greater risk of suffering combat stress and having emotional problems. Those soldiers often pay a profound psychological and emotional toll, according to Shira Maguen, lead author of the study on soldiers and post-traumatic stress disorder. ...
"Those who acknowledged killing somebody in combat were more likely to have PTSD symptoms, anger, relationship problems," said Maguen, a staff psychologist at the San Francisco Veterans Affairs Medical Center. ...
Grant Speakes is among tens of thousands of U.S. troops who say they have had some type of combat stress from their service in Iraq or Afghanistan. Of the nearly 2 million veterans, 5% to 20% have some symptoms of post-traumatic stress, according to the Joint Chiefs of Staff.
A related study on comparable Vietnam War veterans' experience was published last September:
This study examined the mental health and functional consequences associated with killing combatants and noncombatants. Using the National Vietnam Veterans Readjustment Study (NVVRS) survey data, the authors reported the percentage of male Vietnam theater veterans (N = 1200) who killed an enemy combatant, civilian, and/or prisoner of war. They next examined the relationship between killing in war and a number of mental health and functional outcomes using the clinical interview subsample of the NVVRS (n = 259). Controlling for demographic variables and exposure to general combat experiences, the authors found that killing was associated with posttraumatic stress disorder symptoms, dissociation, functional impairment, and violent behaviors. Experiences of killing in war are important to address in the evaluation and treatment of veterans.
Source: "The impact of killing in war on mental health symptoms and related functioning," The Journal of Traumatic Stress, Volume 22 Issue 5, Pages 435 - 443
Details via Steve Tokar, UCSF release:
“Killing, in a variety of ways, turns out to have a wide range of mental health and functioning impacts,” says Maguen, who is also an assistant clinical professor of psychiatry at UCSF. “We knew it would be important, but we were surprised at the extent to which the effects of being in combat faded in comparison.” ...
The authors used data collected during the mid to late nineteen eighties for the National Vietnam Veterans Readjustment Study (1990), a comprehensive U.S. government study of psychological problems among Vietnam veterans who served between August 1964 and May 1975.
They classified respondents into two groups – those who reported killing, or believing that they killed, another person during combat, and those who did not. They then compared the two groups according to various psychological and behavioral measures: PTSD; violent behaviors; functional difficulties in areas such as employment, relationships, legal problems, and substance abuse; and dissociation, a mental state in which a person experiences a sense of separation from his or her own thoughts or feelings. The 47 percent of veterans who reported killing scored significantly higher in all those categories than those who did not.
Maguen stresses that the study was not designed to investigate why veterans who killed had more psychological difficulties than those who did not, and that the results need to be replicated in future studies. However, she says that in her own clinical practice, she has observed that the act of taking a life can have a profound effect on a veteran of war: “In the military, you’re trained to shoot at a target, but sometimes the humanity of that target intrudes, and people come to question what they’ve done.”
The study authors emphasize the relevance of their results for the current generation of troops serving in Iraq and Afghanistan, citing previous research indicating that up to 65 percent of service members returning from the war in Iraq report killing an enemy combatant, and up to 28 percent report being responsible for the death of a noncombatant.
“It’s very important to systematically assess and address the impact of killing,” says Maguen, “and to evaluate it in the most sensitive and supportive way we can.”
While killing in combat appears to increase one's chances for PTSD symptoms, participating in such activity is not the only risk factor for those returning from the war zone.
Others have concluded:
One of the ways the authors have been reminded of the potential discrepancy between their own perspective and the veterans is the surprisingly frequent reports from veterans that much of their suffering is not related to combat or to disability per se, but rather to leaving the military and integrating in a civilian culture. Veterans also often indicate that they perceive honor in their experience and their injuries, and that to some degree suffering shows respect for the fallen and the sacrifices they have made. Hence, appreciating individual meaning is critical for therapeutic alliance and treatment. Metaphorically, for some the experience of combat can be compared with an accelerated aging process: one finds themselves returning from combat with a myriad of physical problems, experiencing suffering, pain, and reduced ability to participate in valued activities, feeling detached from society, and with a special wisdom that has little outlet in our culture. Anecdotally, many veterans return from combat with world experience that they have difficulty reconciling with their pre-combat belief systems, and with an unusual wisdom and perspective that is not necessarily valued by civilians in U.S. culture. As part of a holistic model of treatment, the authors encourage consideration of not only the veteran and their family, but the education of the larger population to be more compassionate, knowledgeable, and appreciative of the sacrifices made by those who serve.
Source: "Post-acute polytrauma rehabilitation and integrated care of returning veterans: Toward a holistic approach," Rehabilitation Psychology. Vol. 54(3), August 2009, pp. 259-269