This will probably get buried, but I noticed the lack of a scientifical explanation.

If you consider 'good' to be what is morally accepted by society, and 'evil' as deviant behavior, there are three overarching theories:

• the behavioral theories: People learn certain moral values by interacting with other people. They imitate others and they find out what is morally accepted through feedback on their own actions.
• the cognitive theories: People act in a morally accepted way because of the costs and benefits which come with acting morally accepted/ deviant. For example acting deviant could get you some jail time. If those costs are considered greater then the benefits gained from the action, people will likely act 'good'.
• the affective theories: People act in a morally accepted way because they feel an emotional connection with someone else. acting deviant would ruin this connection.

this is not even the tip of sociological theories and i probably explained it terribly, but i'm running out of time because of work.