On the one hand, you have things like psychopathy, sociopathy, and malignant narcissism. From what I've read in articles (and seen in documentaries), these traits result from a combination of genetic vulnerability and environmental factors, e.g., growing up with abuse.
On the other hand, even normal people have tendencies to act in a way that is unconscionable. Take the Standford Prison Experiment, for instance.
Stanford prison experiment - Wikipedia
The students involved in this experiment that filled the role of prison guards were healthy and mentally sound, but even so, they resorted to sadistic behaviour. They tormented and humiliated the students who acted as prisoners.
The Milgram Experiment is another good example of this inclination.
Milgram experiment - Wikipedia
When asked by authority figures in lab gear to administer a fatal electric shock, the majority of participants complied.
The conclusion I've drawn is that many of these reprehensible acts are, at their root, caused by either having authority over others or answering to an authority figure.