Evil: What is morally or ethically reprehensible; what is very bad.
That there is such a thing as an evil act has been evident to most men, and that it has a lot to do with the human capacity for being harmed or hurt, and suffer pain, starvation or persecution, but what was and was not considered evil has varied a lot.
1. Evil and Our Side: Possibly the most evil thing about human beings is that evil becomes good if it is committed by our side:
"Actions are held to be good or bad, not on their own merits but according to who does them, and there is almost no outrage - torture, the use of hostages, forced labour, mass deportations, imprisonments without trial, forgery, assassination, the bombing of civilians, which does not change its moral colour when it is committed by 'our' side." (The Collected Essays, Journalism and Letters of George Orwell, vol 3, p. 419, written in May 1945.
2. Evil and indifference: Also, while it is not so very difficult to know what harms, hurts, or pains another human being, it is remarkably easy for humans not to care, especially if it concerns the suffering of someone not in their own group:
"The only thing necessary for the triumph of evil is for good men to do nothing." (Edmund Burke)
This is why there has been a lot of evil in the world: If there were good men who might have prevented it, they were in a minority, and either did little or nothing, or were killed or otherwise disabled by the majority before they could do much, or else simply did nothing because 'video meliora proboque, deteriora sequor' (Ovid) is the ordinary way of the human heart. (For those who don't have sufficient Latin: 'I see the better and approve of it, but do the worse'). Even so, it is well to note that
"A person may cause evil to others not only by his actions but by his inaction, and in either case he is justly accountable to them for the injury." (John Stuart Mill, On Liberty)
In any case, the fact is that most men are prone to benevolence only to those human beings they count as belonging to their own group, and to indifference or malevolence to all others, unless there is some sexual motive: It is tends to be easy to like and forgive a beautiful person.
3. Evil and unhappiness: Yet another reason why evil is important to human beings, even if it is the pain and misery of outsiders of one's own group that one feels indifferent to: The evil that men do is caused by their unhappiness.
A person who feels truly happy, for whatever reason, good or bad, does not feel an inclination to harm or hurt another person (some rare perversions excluded), and does not feel much of an inclination to act on feelings of anger or irritation; a person who feels unhappy usually feels some inclination to make other persons suffer, if only to feel oneself less unhappy than one makes the others feel.
4. Evil and human suffering: Since there are many who hide their conformism, hypocrisy, or lack of true intelligence and individual courage under the pretense of being post-modernists or relativists, it may have some use to point out that there is - except perhaps if you are insane, a severe autist, or otherwise intellectually or emotionally much handicapped or harmed - very little that is 'relative' about human suffering, and also very little that is difficult to understand about it. However, I grant is has turned out to be all-too-human to be indifferent to or gloat over the pain, misery, suffering or unhappiness of other persons, that do not belong to one's own group.