We live in a world that glorifies bad behavior. Neural science can trace the electrochemicals (self-manufactured chemicals or internal chemicals) that flow through our system when we behave in ways that are harmful. The effect is a temporary good feeling when we behave poorly.
The external manifestation of this behavior is rationalization.
You hit my sister because she hit you. You feel good about hitting your sister because your mind was able to manufacture a chemical that covered the pain (electrochemical) that is the natural consequence of behaving poorly. After a period of time, the effect of the behaviorally induced drug wears off and you feel guilty.