When people think about the depths of depravity that their fellow human beings are capable of, people are constantly surprised by various deviant acts that they hear about or see on television. Part of this is due to most people having an a historical memory of violent acts that people commit during war. Another reason for this is that unless an event directly impacts people, their memory does not extend past a few hours. Modern society has geared people toward a delusional sense of civility thinking that at our core, human beings are closer to angles than devils. People often intentionally ignore an essential part of human nature because acknowledging evil will create a sense of discomfort that people would rather avoid. Many people believe that ignoring a problem makes it go away, which is naïve, but it helps people sleep better at night.
Everyone has a different and subjective standard on what evil is, and when certain acts cross the threshold and civility is broken some action(s) make a person beyond redemption. Believing the best about our fellow human beings makes one feel noble and enlightened, but there are both positive and negative effects when generalizing human beings. Thinking about people in positive terms allows us to be blinded by barbarism when a person commits an obscene act against another person. Thinking negatively about other people makes us worse on the inside because seeing the best humanity has to offer and the joys of companionship will not be seen; thus keeping people from connecting with each other to improve the condition of mankind.
Who or what is considered evil is can take on an objective quality if a broad, yet opaquely defined societal consensus does exist on a definition of evil. In most cases, evil is whomever the public mob wants to go after if a person or group action(s) are considered morally reprehensible and recorded for posterity. Symbolism flowing into real life, anything associated with the color black is seen as evil while conversely, anything associated with the word white is seen as pure and innocent to the point of blindness. Associations with color being good or evil is not limited to the ways fellow human beings see each other, but in the use of everyday language. White lies are not seen as a bad thing because supposedly the intent behind them is good. Black Friday is probably the only exception to anything labeled black being a bad thing. Not to say that perceptions of color related to good or evil is true, but those associations do shape our views and the decisions that people make.
The biggest problem with evil being subjective is that some bad outcomes are not considered evil, but deserve to be highlighted for the inimical effects that people feel. People can feel evil, but feelings are usually subjective and lacks a rational basis, therefore making evil something that emanates from an individual world view. In the collective mind of society, we have a hard time thinking of an attractive person as evil and an unattractive person as good, but only a conscious effort can correct such biased thinking. Preconceived notions of evil related to appearance explains why people are often surprised when seemingly good people commit atrocities. Evil is shaped by our experiences, and more so, by our expectations, if people are looking for evil they will find it in places where evil did not exist before.