Being and nothingness. The title of Sartre's major work in philosophy. 

"Existence precedes essence," which means that for human beings, there is no inherent nature or reality that defines us – we are not created to know and love the Creation and our Creator, to perfect our inborn gifts, for example; or to perfect our nature by becoming virtuous. Unlike inanimate objects and other life forms, humans do not have a predetermined nature or essence that defines them. Actually, nothing does except in the sense that science can describe it. The universe for science is a closed, deterministic system; events can be predicted in the future or described in the past because everything is based on cause and effect.  We, however, exist first and then define ourselves through our choices and actions. Through self-consciousness we transcend nature itself; we are little absolutes. We determine what our reality is, and thus also what is good and evil; it is not fixed or given. This freedom is not just the ability to make choices but entails the responsibility to choose and create one's own values and meaning in life. With freedom comes anxiety and a sense of abandonment because we bear the burden of creating our own essence. The anxiety leads us into “bad faith”: we deny our freedom and seek to become like a thing or an animal with a fixed nature: e.g. by conforming to predetermined roles or adhering to social expectations. This necessarily involves self-deception given what Sartre posits about our inherent lack of reality. Our nature is to have no nature. That is the thesis Sartre develops in his long book – and develops perhaps even better in his literary works. 

     For the wise prior to Sartre and modernity, evil was the failure to bring the mind and heart in tune with reality - not by instinct, as with the other animals, but through education and effort - and for Christians, grace. Our nature was open in a way that was not possible for other animals, but it was not radically open as it was for Sartre. We are not little absolutes, but finite and fallible. We are creatures, not in the radical sense creatures. The universe is meaningful: the value of anything attaches to its reality. The Good is what accords with reality. Our task is to gain wisdom - insight into reality - and make it possible to want and act so as to conform our actions and way of life to objective reality. We are free not to do that, but that is folly - or sin.

   There are two forms of sin or evil - both contrasting with Sartre's belief that evil was giving into bad faith i.e. not embracing your freedom (understood as autonomy) to create your own values. Loving something real, and thus good, in excess such that essential connections are damaged – food is good but loving the pleasure of eating (being addicted) can destroy your health, your bonds with other people, your connections to the earth; it can break the wholeness of your body-soul. If you elevate your narcissistic desires or deep insecurities over the good things in your life or indeed over all of Creation (here I can’t help but think of Trump, addicted to himself and his fantasies), you “love” something good, namely your life, but in a perverse way that damages others, again destroys essential connections to others, the common good, God, your own soul. I think people who get off on feeling self-righteous by judging others – contrary to the teachings of Christ – are guilty of a perversion of love: not of the people they cut out through their judgments but of themselves and their group, which they elevate over their fellow man. These forms of diseased love are all-too-human, and have their roots in damaged egos. They are failures of love. I guess most all of us fail at love in different ways; thus, 'let he who is without sin cast the first stone.' But though we may not judge, we are obligated to do what we can to love, honor, and cherish - and preserve - what we know in our hearts to be good. 

     The second form of evil is more sinister: the willful desire that what is real, what exists, and thus is good, essential connections included, should cease to exist or be annihilated. Or at least be corrupted into something not good: as elves were perverted into orcs by Sauron in Lord of the Rings; as a meadow  is perverted into a garbage dump or another Walmart; as the great plains were corrupted and made into the chemicalized monocultures of agribusiness; as an innocent child with many gifts of mind and a potential to become a loving person is slowly deformed into a relentless ego-and-greed-driven hedge fund manager. This is a demonic form of evil, perhaps best expressed by an imagined demon, Goethe’s Mephisto, who replied when asked to say who or what he was: “Ich bin der Geist der stets verneint! /Und das mit Recht; denn alles, was entsteht, /Ist wert, daß es zugrunde geht;/Drum besser wär’s, daß nichts entstünde…” (I am the spirit that negates. / And rightly so, for all that comes to be/Deserves to perish wretchedly; ‘Twere better nothing would begin…]    

     If it is better to be than not to be, if the act of existing is itself inherently good, then evil can only be perversion or negation. It has no positive content - like Sartre's autonomous man with no pre-given reality. It can’t create anything. Only destroy or disconnect. And you know it not by how it presents itself but by its fruits. (I learned this from St. Augustine’s Confessions – I disagree with Augustine about many things, but he hit that nail right on the head. The theme is very much a part of Tolkien’s Lord of the Rings as well.) 

     Sartre's picture of humankind is the perfect expression of one side of modernity (the other being a naturalism that leaves no conceptual space at all for freedom). The Sartrean hero finds a perfect literary expression very early in the modern era: it is Milton's Satan: The mind is its own place, and in itself Can make a heav'n of hell, a hell of heav'n.