The question of what evil is a perennial one. I have seen a long and sometimes heated discussion about it on a list that is concerned with the poetry of T. S. Eliot. One participant states emphatically that evil cannot be defined because there is no common agreement on whether or not something is evil. He points out as an example that slavery was not considered evil until relatively modern times. The slaves who were involved in Spartacus' rebellion were not opposed to slavery in general but only to their own servitude. If they had made good their rebellion and made new lives outside of Italy they would likely have kept slaves. Others point out that there is evil and that torture is an evil practice. I do not see how this meets his objection because torture has a long history and is commonly practiced.
To me, their discussion of what is evil fails because they do not begin with the essentials. In what cases can it be said that evil exists. Killing is regarded as evil but if a lion kills a gazelle their can be no question of the act being evil. It is the nature of the existence of these two species that they interact through a predator-prey relationship. Each species benefits from this. It is part of the Platonic idea of each of them.
Part of the Platonic essence of humanity is the nature of social cooperation. Humanity has evolved as a social species. Human Social evolution has discovered social cooperation and the advantages that it confers. Humans have been selected both physically and socially to cooperate in a large society. Experiments have shown that dogs have an innate sense of fairies. Humans have a similar sense of fairness and will recognize breaches of this expectation.
Cooperation produces this sense of fairness and this cooperation is also the basis of society.
This provides the basis for a concept of evil. Evil can exist only within a background of a cooperative society. Evil is a characteristic of a social strategy that deny the benefits of cooperation to others.
The T. S. Eliot discussion is premature. They cannot discover evil because they have not defined how it can come into existence and the reason for its existence. They presume that evil exists a priori but fail to see that evil is something that did not exist before it was discovered. It has no individual Platonic essence because it cannot exist without the cooperation that gives it its advantages.