From the concluding paragraphs of Shane O'Mara, The interrogator's soul, Aeon:
And significantly, the most empathic interrogators are also the most vulnerable to terrible psychic damage after the fact. In his book Pay Any Price (2014), The New York Times Magazine correspondent James Risen describes torturers as ‘shell-shocked, dehumanised. They are covered in shame and guilt… They are suffering moral injury’.
A natural question is why this moral and psychic injury arises in soldiers who, after all, have the job of killing others. One response might be that the training, ethos and honour code of the solider is to kill those who might kill him. By contrast, a deliberate assault upon the defenceless (as occurs during torture) violates everything that a soldier is ordinarily called upon to do. Egregious violations of such rules and expectations give rise to expressions of disgust, perhaps in this case, principally directed at the self.
This might explain why, when torture is institutionalised, it becomes the possession of a self-regarding, self-supporting, self-perpetuating and self-selecting group, housed in secret ministries and secret police forces. Under these conditions, social supports and rewards are available to buffer the extremes of behaviour that emerge, and the acts are perpetrated away from public view. When torture happens in a democracy, there is no secret society of fellow torturers from whom to draw succor, social support, and reward. Engaging in physical and emotional assaults upon the defenceless and eliciting worthless confessions and dubious intelligence is a degrading, humiliating, and pointless experience. The units of psychological distance here can be measured down the chain of command, from the decision to torture being a ‘no-brainer’ for those at the apex to ‘losing your soul’ for those on the ground.
Compare this with these remarks on the difficulties mass killers face when confronting victims who never did them any harm and are not violent toward them:
Any kind of violent confrontation is emotionally difficult; the situation of facing another person whom one wants to harm produces confrontational tension/fear (ct/f); and its effect most of the time is to make violence abort, or to become inaccurate and ineffective. The usual micro-sociological patterns that allow violence to succeed are not present in a rampage killing; group support does not exist, because one or two killers confront a much larger crowd: in contrast, most violence in riots takes place in little clumps where the attackers have an advantage of around 6-to-1.
That's by sociologist Randall Collins, whom I'm quoting in a post, Rampage Killers, Suicide Bombers, and the Difficulty of Killing People Face-to-Face.