Ethics consists in the study of the good and the evil, this is the currently accepted definition of ethics. The beginning of the study of ethics begins with Aristotle and the Greek philosophy. In Nicomachean Ethics Aristotle argues that the activity of man tends to happiness and the ethics is part of the politics. For Aristotle there is no universal definition of the good: the good of war is victory, the good of medicine is health.
The Gospels observe the idea of good and evil in another way. In the Old Testament the Book of Psalms begins with the poem about "The two roads"; the Gospel of Luke retake the teaching of the two ways in the 4 Beatitudes (Luke 6: 20-23) and the 4 curses in Luke 6: 24-26.
The Gospel of Matthew teaches us: "No one can serve two masters," Matthew 6:24; also, "for those who live according to the flesh set their minds on the things of the flesh, but those who live according to the spirit set their minds on the things of the spirit", Romans 8: 5. Thomas Aquinas, father of the Church, summarizes: We sin or live righteously.
The Gospel tells us that there are two ways for man, as in the Parable of the Good Samaritan: we act like the Levite and the Pharisee who pass by in front of our neighbor or  we act with righteousness and mercy like the Samaritan, Luke 10.
Basically good and evil are discernments, mentalities, one mentality is benevolent, edifyng, progressive  this mentality is call "good mentality." There is another that is destructive, chaotic, unfair, we call this "bad mentality". We all discern, we choose, this is the starting point.
The origin of these two ways or mentalities as the gospel teaches us, are the truth and the falsehood. Truth and falsehood are previous concepts than good and evil: "who practices the truth comes into light", John 3:21. The truth belongs to the genre of the being, is the precise and the exact. Falsehood, on the other hand, belongs to the genre of the non-being. In the book Metaphysics, Aristotle explains that when two opposites belong to different genres there is no intermediate state. Something is true or false, there is no intermediate, as is the case of temperatures, we have an intermediate state between cold and hot, which is warm.
We live according to the spirit, that is, we live righteously; or we live according to the flesh, that is, we sin, as the Gospels teach us. In this sense there are no "grays", things are either "white" or "black".
We hit or we fail, we are false or truthful. In this consist the study of ethics.