Thirty-one soldiers who had killed while in combat and who had subsequent psychiatric examinations were asked about acts of "personal violence" in Vietnam (acts against persons at close range judged to be unnecessary from a military point of view). Fourteen reported engagement in personal violence, while nine others reported witnessing such behavior.
In this group, all who reported participation in personal violence had volunteered to serve in Vietnam. Significantly more participants reported killing four or more persons in Vietnam than did nonparticipants, and the participants more frequently had a history of arrest prior to military service. As a group, participants could be distinguished from nonparticipants by the average number of positive items present in each of several groupings of precombat variables. Personal violence in combat results from an interaction of individual, group, and situational factors.
Yager J. Personal Violence in Infantry Combat. Arch Gen Psychiatry. 1975;32(2):257-261. doi:10.1001/archpsyc.1975.01760200121012