Violence can cause personality change and military combat is among the most intense. One pesonality change, called the "heart of darkness" syndrome, transforms a previously normal person into an enjoyer of killing, exceeding the normal reactions to prolonged combat though having revealed no antisocial behavior earlier and having a close, caring relationship with their comrades. The personality change from "normal" to "happy killer" occurred after witnessing their comrades' deaths.
Yet this reaction to combat trauma is extreme since soldiers with brief combat experience need not make such a personality adjustment to how they relate to reality in order to psychologically survive, having adjusted to it with controllable fear during their potentially fatal experiences. But continuous combat tends to produce a sense of denial despite the close-by death of comrades, causing a persistant blunting of feelings that can affect relationships long after military service has ended.
Three possibilities exist within the combat continuum of personality change: (1) none, where the ex-soldier remembers and re-experiences past events without denial or the blunting of feelings or becoming a lover of killing; (2) the soldier develops a warrior mentality but functions under orders and kills only for military purposes, retaining a sense of ethics; (3) the soldier has come to love killing, feeling invulnerable and lacking empathy for those whom they kill.
But this continuum of human personality change cannot be so clearly separated since courage, heroism, self-control, and intense comradeship exist in all.
Reference: "Combat and Personality Change" by Samuel L.Bradshaw, M.D., Carroll D. Ohlde, Ph.D., and James B. Horne, M.D. - Bulletin of the Menninger Clinic, Fall, 1993, pp. 466-47