And it seems to me that such a person is clearly stupid and selfish: he, like many others, was not taught a lesson in morality and humanity in childhood (which may not be his fault). Surprisingly, then a person asserts himself even more in his righteousness with age and only poke his nose so to speak, something visual (the death of a loved one) as his own experience may change something. Therefore, it is better to beware of fools and stupidity)
In psychiatry classes, we were told that a person who is capable of killing his own kind, regardless of the motive, is mentally unhealthy.
This is difficult to accept, but usually people who do atrocities are mentally healthy. At least in the same range of mental normality as you and I.
First, they simply have such values, permanent or temporary, within which their actions are normal, fair and even completely righteous. Kill the bourgeois / whites / churchmen. An unfinished counter. Jews. Heretics. Soldier of the enemy. The civilian population of the enemy. Just a civilian population. Children-hostages, well, along with the terrorists.
Secondly, the psyche, in general, is well protected. In most cases, a person will justify himself. It's all "they". For example, "they" themselves ... Dress perversely. They want to take over the world. Serve the devil. They slander our Values.
It is worth clarifying here. If a person kills for pleasure, he is sick. If a person kills for the sake of gain, and at the same time does not feel anything, most likely he is cold-blooded, and does not see anything dangerous and painful in death.
Why do people generally bear killing hard? Firstly, fear of consequences, secondly banal humanism (and the realization that you have taken "everything" into a person), and thirdly, blood intolerance.
But in general, this is a habit. Reading interviews or memoirs of a military man, everyone in one head says that with each death you treat her more and more calmly.
As they say, a habit is developed in 21 days, but here, probably for 21 corpses.