the SEP has an entry about this. You might prefer to just read it, but it's kind of long so i guess i'll pick out some stuff i think might be interesting/relevant
it doesn't seem like there's much agreement on what constitutes evil, although the author of the article is able to pull up some pretty striking examples of actions and individuals that are described as such: the Holocaust, the Rwandan genocide, 9/11, Hitler, Jeffrey Dahmer. Contra @Bee, the entry states that in the broadest sense, we can ascribe evil to objects such as hurricanes, or sensations such as toothache. however when we use the word 'evil' to mean something different, or more severe than just 'very bad', the entry argues that this is a narrower sense which can only apply to moral agents and their actions.
a big question is whether there's any value in having the concept of evil as distinct from bad or wrong. some people have argued that there is. Claudia Card, for instance, designates the gender pay gap as 'bad' and domestic violence as 'evil', and argues that therefore the latter is the more pressing issue. Daniel Haybron is quoted saying "Prefix your adjectives [such as ‘wrong’ or ‘bad’] with as many ‘very’s as you like; you still fall short. Only ‘evil’, it seems, will do" which makes it sound like evil is something qualitatively different from ordinary badness. however it is widely (not universally) accepted that for an action to be evil it must at the very least be wrong. according to Hillel Steiner, a wrong action becomes evil when the perpetrator takes pleasure in performing it. whereas according to the author of the entry, Todd Calder, for an action to be evil the perpetrator must intend to cause significant harm, which is not necessarily the case with wrong actions. Calder argues that evil and wrong share no essential qualities, which seems to imply you could have an evil action that was not wrong.
of course there's the problem of defining what evil is in the first place. this is something that has preoccupied theologians because of the problem of evil, i.e. 'if God is supremely good, why is there evil?' one possible response to this is Manichaean dualism, which sees the universe as the product of two opposing forces, a supremely good God and the Prince of Darkness. this works because in Manichaeanism God is not all-powerful, but doesn't work in Christianity, where God is supposed to be omnipotent (there's the familiar challenge of: omnipotent, omniscient, benevolent, it seems like you can't have all 3 and must pick 2), and from a secular standpoint it's obviously unsatisfying as a definition of evil since it entails a supernatural origin. the Neoplatonist answer is to define evil negatively, as privation of good. this *kinda* works but isn't very satisfying because it seems pretty easy to come up with instances where privationism isn't sufficient explanation: a toothache is not just 'not pleasurable', it hurts. a torturer who takes pleasure from hurting other people isn't merely lacking kindness, she wants her victims to suffer. it seems that evil has a positive quality, it's not simply an absence. the entry refers us to a paper by Anglin and Goetz for the privationist response to this criticism, i don't have a subscription to JSTOR but apparently the gist is that they consider pain itself to be neutral, and only evil insofar as it is a privation of "our normal state of bodily well-being". w/o reading the piece i can only speculate but maybe this is in the sense that pain is actually an evolutionary adaptation to prevent injury? substitute God's design for evolutionary adaptation if you like
(in support of this argument they cite a passage by Thomas Aquinas in which he states that it is not an evil for a person not to have wings, but it is an evil for a person not to have hands or a bird not to have wings. i wonder what the father of Thomism would have made of otherkin)
the SEP entry goes on to supply some more recent theories of evil. for Nietzsche, good and evil are concepts which must be rejected because they stem from an unhealthy impulse of the weak to condemn the strong, and ultimately from a worldview that prioritizes relief from suffering over creativity. Against this view, Claudia Card has argued that judgements of evil constitute the healthy recognition of injustice.
for Kant, evil is anything human beings naturally incline towards which is not fully good. Kant identifies three degrees of evil, from least to most bad: frailty (being too weak-willed to do the right thing), impurity (doing the right thing for partly self-serving reasons, and therefore being guided by something other than moral law), and perversity or wickedness (acting only in self-interest, w/o caring if it's the right thing or not). for Kant, an agent with a perverse will is evil, whether or not they do the right thing. so for Kant, telling the truth to enhance your own reputation is just as evil as torturing someone for kicks. Kant also has a concept called 'radical evil' (that is, evil that resides in the radix, the root) meaning that evil is inherent to human nature.
Hanna Arendt uses 'radical evil' differently. for Arendt, radical evil is wrongdoing that cannot be captured by other moral concepts, such as the Holocaust. a characteristic of radical evil is that it reduces human beings into living corpses without spontaneity or freedom. Contra Kant, radical evil for Arendt cannot be done for humanly comprehensible reasons such as self-interest. it is a means of reinforcing totalitarian control by demonstrating that all things are possible. Arendt is also famous for her concept of the 'banality of evil'. this is demonstrated by Nazi pen-pushers who condemned people to death through sheer thoughtlessness. for Arendt, then, an evil person is no different from anybody else, but who does monstrous actions w/o really thinking about them. note that this is incompatible w/ Steiner's conception of evil as having only 2 components, pleasure and wrongdoing.
the article goes on to discuss some specific cases, e.g. it is not evil to care about your clothes, but Eve Garrard argues that it would be evil to refuse to rescue a drowning child because you didn't want to get your clothes dirty. Garrard's conception of evil rests on a distinction between psychological and metaphysical silencing. A metaphysical silencer is a reason so morally weighty (that child is going to die) that it renders other reasons trivial. A psychological silencer is the same thing, but it doesn't carry moral weight, only weight as pertains to your personal interests (my clothes will get wet). When these two come into conflict, and an individual chooses to prioritize the psychological, that's evil.
there are various instances where we might want to say that an action wasn't evil due to something about the agent (are psychopaths criminally insane? is a bad upbringing an excuse, what about ignorance?). the case of sadistic voyeurism is particularly divisive. a voyeur does not cause suffering, and may not be able to prevent it. Haybron gives the example of a sadistic quadriplegic who cannot communicate, but wishes "nothing more than the greatest suffering for her fellow creatures". the quadriplegic cannot be accused of allowing harm to come to other people, so it is not clear that she is guilty of any evil action, but we still might incline to call her an evil person. note that this example still involves harm befalling a person, even though the presumed evil person is not responsible for said harm. so there's the question of *how much* harm must be caused for an action or person to be considered evil. John Kekes argues that the harm must be "serious and excessive". this strikes me as pretty vague but w/e.
this post is longer than it was gonna be, i'll just throw out a few more definitions and call it a day. Terry Eagleton defines evil as the desire to annihilate all being, which does sound pretty evil. similarly Phillip Cole, who is sceptical about the validity of the concept, defines it as the destruction of others for its own sake. for Kekes an evil person does evil frequently, whereas Haybron argues that if an individual rarely performs evil actions but only because they are too much of a coward to perform those actions very often, that individual would still be evil. for Luke Russell an evil person is one who is strongly and fixedly disposed to perform evil actions when they can get away with them. Colin McGinn defines an evil person as "one that derives pleasure from pain and pain from pleasure" which kind of reminds me of the relationship of D&D's undead to the living. that's enough from me i think, read the entry if you want more.
Comments
Also watch the good place