My own ethical approach to good and evil is that people are born with a set of individual rights, that cannot be taken away from them. Rights such as the right to life, free will, and the right to own personal property. People that follow these rights are automatically put into the "good" category (X). But, when you violate someone else's rights, you leave group (X) and move to group (Y), and henceforth, become an "evil" person. Granted, the levels of evil are not all exactly the same; if you kill someone you are more "evil" than if you shoplift something. Therefore, remaining in the (X) category means you are essentially a good person because you haven't violated the rules, and thusly, haven't descended from being an (X) into being a (Y). You are good automatically because you aren't a (Y). If you commit a crime, you can only go from a (Y) to an (X) by paying for your crime, an eye for an eye. My theory of ethics and good vs. evil closely resembles John Locke's Social Contract Theory. However, mine also includes the theory that when you commit a crime like murder, you announce to the world that you think murder is ok, and therefore it would be allowed for law-abiding citizens to kill you, because murder is acceptable for you. In addition, committing a crime, and therefore announcing that you think that crime is ok, means that you have descended into the (Y) category, where you remain until you pay off your debts.
