The book of Genesis introduces the story of Adam and Eve, and how they fell from grace due to consuming the fruit of knowledge, and with it the knowledge of good and evil. It isn't specified exactly when this occurred, but one can presume that it coincided with the cognitive awakening of humanity and sparked the era of civilization. Modern homo sapiens existed long before the awakening of their cognitive faculties, as hunter-gatherers foraging for food, and it isn't specified what the fruit exactly was, perhaps a psychoactive substance.
Everyone has the capacity for evil, it's innately built-in to human nature as Kant would suggest. Kant laid the groundwork for metaphysical rationale for morality, but even in this work we see that morality is inseparable from the teleological argument, that is a morality that is justifiable without God. The implication is that morality can't be justified by secular ideals alone, such as the non-aggression principle, even natural law has to be based upon Nature or Nature's God.
As modern civilization is increasingly secular, even commodifying spiritual practices such as meditation for mental health benefits in order to remain productive, I think that a state of anomie is the inevitable conclusion of where society is headed. People are less empathetic, less introspective of their own actions, or even given up on making moral distinctions, because they think it does not matter. We see further and further overlap of legal and immoral acts, as usury, scams, and theft are normalized. Healthcare insurance in the USA is one of those scams which is legal theft; or university education these days which is rife with academic fraud and worthless degrees. Taxation itself is legal theft, backed by state-sponsored violence.
Modernity is plagued with moral grey areas, and it's that ambiguity that leaves us all wondering which one of thousands of pages of legal codes one could be violating at any moment. Without a sense of good and evil, one condemns both the robber and the robbed, as neither have the moral high ground. As humanity veers further into secular governance, we condemn both good and evil, which will allow evil to prevail.
"For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms." - Ephesians 6:12
Defining evil
The psychiatrist M. Scott Peck had an interesting definition for evil: as a militant self-denial of guilt. An evil person has a conscience, which they actively self-deceive in order to protect the ego, which is different from a sociopath who lacks a conscience. In this definition, evil starts by lying to oneself, and by extension to other people. An evil person has the desire to project their sins onto certain targets.
I have some qualms about this definition, mainly that it can be used to vilify the weak of mind. Human nature is sinful, even the most good person is not without sin. In Joost A.M. Meerloo's The Rape of the Mind, he describes how one's own conscience can be weaponized against oneself, using psychological warfare tactics such as gaslighting, or Kafkatrapping, a person's inherent sinful nature is wrongly assumed to be indicative of greater guilt for evil acts. This is how false confessions are brought about, it forces a false dichotomy of being evil vs. admitting guilt. It also makes casting judgement on evil difficult, because doing so without having the advantage of authority or large numbers does more harm to the accuser.
Defining evil is tricky, because it often assumes a priori knowledge of what is good or what is evil. Hence why definitions often fall back to religious texts, such as the Ten Commandments, which are by no means an exhaustive list. I think that since there is great difficulty in deriving a secular definition of evil, one must include theology to get a holistic definition of evil.
I think that evil is rooted in deception, not just in telling lies, but a sort of metaphysical deception. The greatest lie that the Devil ever pulled was convincing the world he didn't exist, which describes the state of the world accurately. The secular religions that replaced God, such as techno-capitalism, have no foundations to guide humanity towards moral behavior. Philosophers such as Nietzsche tear down any dogmatic basis for morality in Beyond Good and Evil, but offer nothing to replace it with.
Evil is embedded in human history, after all it is curious how the good guys have won every war in history. It's a myth that the truth will win in a battle of ideas. The truth is often ugly and complex, and subject to market forces, truth is unmarketable and outsold by untruths.
Peak evil
Humanity itself is in a bubble right now, with a world population of over 8.2 billion people. If we assume that 1% of the population is irredeemably evil, that means there is more evil in this world than ever existed. The most common archetype of evil, petty criminals with a poor upbringing, isn't terribly consequential nor interesting. Evil has a tendency to take root in positions of power: power corrupts, and absolute power corrupts absolutely.
The 20th century was one the bloodiest in human history, with over 200 million deaths attributable to governments. While political leaders have caused millions of deaths indirectly, actual murders were carried out by the rank-and-file, just following orders. Hannah Arendt characterizes evil as not extravagantly wicked, but banal: evil acts are carried out by average, mundane people who are motivated by self-interest and lacking in critical thought.
Secular humanism which has replaced God in modern society only offers insincere and unenforceable legal doctrines: we got the Universal Declaration of Human Rights and nuclear bombings in the same decade from the same government. People are more concerned with projecting goodness via virtue-signalling, than actually being good, which carries the risk of being harmed.
The people who are leading humanity into mutually assured self-destruction are irredeemably evil, it's no surprise that the populace may readily believe in conspiracy theories (which many inevitably do turn out to be true). The refusal or inability to cast out evil in positions of power may be the downfall of human civilization as we know it.