An article in Scientific American reveals that at least some soldiers actually enjoy killing, particularly when they're not at much risk of being killed themselves.
The article recalls one, "Sgt. Eric Schrumpf, a Marine sharpshooter, saying,
"We had a great day. We killed a lot of people." Noting that his troop
killed an Iraqi woman standing near a militant, Schrumpf adds, "I'm
sorry, but the chick was in the way."
Does the apparent satisfaction—call it the Schrumpf effect—that some
soldiers take in killing stem primarily from nature or nurture? Nature,
claims Richard Wrangham,
an anthropologist at Harvard University and an authority on
chimpanzees. Wrangham asserts that natural selection embedded in both
male humans and chimpanzees—our closest genetic relatives—an innate
propensity for "intergroup coalitionary killing" [pdf],
in which members of one group attack members of a rival group. Male
humans "enjoy the opportunity" to kill others, Wrangham says, especially
if they run little risk of being killed themselves.
Several years ago, geneticists at Victoria University
in New Zealand linked violent male aggression to a variant of a gene
that encodes for the enzyme monoamine oxidase A, which regulates the
function of neurotransmitters such as dopamine and serotonin. According
to the researchers, the so-called "warrior gene" is carried by 56
percent of Maori men, who are renowned for being "fearless warriors,"
and only 34 percent of Caucasian males.
But studies of World War II veterans suggest that very few men are
innately bellicose. The psychiatrists Roy Swank and Walter Marchand
found that 98 percent of soldiers who endured 60 days of continuous
combat suffered psychiatric symptoms, either temporary or permanent. The
two out of 100 soldiers who seemed unscathed by prolonged combat
displayed "aggressive psychopathic personalities," the psychiatrists reported. In other words, combat didn't drive these men crazy because they were crazy to begin with.
...The reluctance of ordinary men to kill can be overcome by intensified
training, direct commands from officers, long-range weapons and
propaganda that glorifies the soldier's cause and dehumanizes the enemy.
"With the proper conditioning and the proper circumstances, it appears
that almost anyone can and will kill," Grossman writes. Many soldiers
who kill enemies in battle are initially exhilarated, Grossman says, but
later they often feel profound revulsion and remorse, which may
transmute into post-traumatic stress disorder and other ailments.
Indeed, Grossman believes that the troubles experienced by many combat
veterans are evidence of a "powerful, innate human resistance toward
killing one's own species."
In other words, the Schrumpf effect is usually a product less of nature
than of nurture—although "nurture" is an odd term for training that
turns ordinary young men into enthusiastic killers.