Evil is the external view of a behavioural pattern which seems to be built-in to primates. It is sometimes characterized in terms of sin, most famously the seven deadly sins: greed, envy, wrath, sloth, vanity, lust and gluttony. Modern technological society seems to be based on these seven principles, plus an eighth principle, passing the buck.
Evil behaviour is often considered to be that which is carried out with little or no regard for the (often intentional) pain inflicted on others. However, all human behaviour is carried out this way, ultimately, if we consider others far outside our personal social loop, or creatures of other species. So an event or process or thing is usually only considered evil when it becomes so extreme as to violate a faction's agreed upon threshold of acceptable ignorance and cast light on ourselves. For instance, most child molesters are parents or near relatives. The "poaching" behavour of a sexual predator who is a genuine stranger is thus regarded as particularly evil, in part because it brings scrutiny on those closer in a child's family, who may be doing the same. Hating and prosecuting the relatively rare paedophile is simply one way that a community assures itself that it is "better" - the pain inflicted on children by the highly traumatic process of adult investigation, and on the adult who is by most definitions simply a sick and compulsive person, is simply not regarded. For these reasons many social workers advise simply not reporting such cases - the evil that results from the system "working as it should" is just too high.
This represents a shift away from trust in centralized systems or judgement. Historically "agreed upon" standards are set by one party and physically forced on another who is usually forced to trust the authority of a moral code, along with their own personal judgement (or terror) of what will occur if they do not obey. For example, the sins above are the product of Roman Catholic thought, which was historically part of the Christian traditions. Of course in their turn the Christians were thought to be evil for refusing to worship the Roman Emperor as a god (Christians were similarly persecuted in Japan). Today it is unpopular to claim that there is any one definition of evil that humans can objectively assess and apply.
Still, some claim evil can be objectively and globally defined and that at least some of what is not evil could be detected by some audit. How fair could such a definition be? At the very least it would have to satisfy participants and observers that their own ideas of what is safe and done are respected. Factions make it impossible to settle on one strict and global definition, so this is a contested term - see glossary for other such contested terms. Probably it is best to consider evil to be factionally defined.
See also: extreme standards