Why is it that there are dozens upon dozens of books about the young worker who dares to protest against dangerous factory conditions while there are practically none about the greedy boss who benefits from keeping him in the dirt? Both figures are equally representative of the human race, after all; overworked laborers have existed since the beginning of history, and so have their masters. And the real-life analogues of these two eternal tropes are all true people. They are all human beings, each with their own distinct memories, emotions and desires.
Another peculiarity about human nature is that we cannot really do anything without justification. Aspiring murderers are generally either ignorant or dismissive of the suffering they will cause; if they knew it, they would abandon their efforts.
Members of Homo sapiens are incapable of bearing the burden of doing evil; we will always try to justify it, whether by external reasons (“For the cleansing of my country!”) or internal ones (“Everyone is horrible to me; he’s so happy, I’ll take it out on him.”) Once these reasons fall apart under scrutiny, we either latch onto new ones feverishly or must suddenly bear the heavy load that is guilt. And guilt is a very hard load to bear: it has caused suicides, murder-suicides, material and bodily sacrifices, dissolutions of entire countries; it has told feverish lies, revealed gruesome truths.