We can intuitively feel when something is morally wrong. When we see an innocent person getting beaten and mugged, our gut tells us that we are witnessing immorality. When we witness indiscriminate mass killing, we feel it in our gut that this situation is evil. When an elderly couple gets scammed out of their retirement savings, we feel angry because we know that something wrong has happened.
Of course, feelings alone cannot objectively tell us right from wrong. If your boss demands that you work harder, and you feel under appreciated and underpaid, you might feel resentful towards your boss. It is very easy for you to feel that your boss is, in fact, evil. You feel like he is exploiting you. You feel like a slave. Do your feelings actually correspond to reality in this scenario? No. You're not a slave. You don't have to do anything for your boss. Just leave.
Clearly, feelings alone cannot help you identify right from wrong. If feelings alone were sufficient, then anybody who feels resentful towards you for whatever reason could make the claim that you are immoral. You probably wouldn't feel good about that. In fact, you wouldn't like that at all. So does this mean the person who feels resentful towards you is also immoral? Are they immoral because they have identified you as immoral? In a world where feelings determine right from wrong, the contradictions are endless.
So what is it about certain actions that make them immoral? What is it about murder that makes murder wrong? Is wrong to kill somebody in self-defence? If you had no choice, then we intuitively feel that killing in self-defence is justified. What about a duel to the death? If you and your opponent agree to a duel, and you kill your opponent, did you commit a moral crime? What about war? Are soldiers evil for killing other soldiers and causing civilian deaths? What about euthanasia? Is ending the life of a terminally ill patient wrong? Given that the context changes the moral implications of murder, the act of taking a life in itself cannot be what's wrong.
What about theft? Surely, it's wrong to steal, but what about confiscating the property of somebody who has made a career of theft and fraud? What about gambling? Surely, if you win all of your opponent's money, he wouldn't be happy with that. You took his money and he didn't want to lose his money. Clearly, the act of taking somebody else's property in itself is not wrong.
What about assault? If you hit me unprovoked, we intuitively understand that is wrong. What if I hit you back? Is that action still wrong? What if we're professional boxers, and we're in a boxing match? Is the act of hitting still wrong?
A pattern is beginning to emerge. Something is morally wrong when you do something to someone else, but you won't allow him to do it back to you. This is the basis of moral universality. Universality means that you may not treat somebody else differently than you would want them to treat you. If you hit me unprovoked, and if you simultaneously prohibit me from hitting you back, that is wrong. That is a violation of universality. In order for you to no longer be in the wrong, you must allow me to hit you back. Thus, when I hit you back, I am erasing your 'wrongness' or 'sin' for lack of a better word. In the case of a boxing match, we have both established that it is okay for us to hit each other. Thus, universality is not violated. There is nothing wrong with being in a boxing match.
Universality does not mean that we have to immediately mirror each other's actions without taking context into consideration. For example, if I am your boss, and I fire you, universality does not mean that you suddenly have the power to fire me. Universality means that I cannot claim that it is immoral for my boss to fire me while I simultaneously permit myself to fire others. If you ever become my boss, I can't complain if you fire me.
Let's go back and revisit murder. Murder is wrong if you allow yourself to kill other people while simultaneously prohibiting others from killing you. In order to correct this wrong and restore universality, you must either not commit murder, or if you do, you must allow other people to kill you. Therefore, a duel to the death does not violate universality because the duelists agree on a common rule – we may kill each other. Soldiers in a war between nation-states are not in violation of universality as long as they don't kill civilians. Soldiers agree that the enemy is allowed to shoot back. Civilians do not have such an agreement, therefore it would be a violation of universality for a soldier to kill a civilian.
A murderer is someone who allows himself to kill others while simultaneously prohibiting others from killing him. This is a violation of universality. In order to restore universality, other people must be allowed to kill the murderer. Killing the murderer does not further violate universality, in fact, this would restore universality. Therefore, it is not immoral to kill a murderer. If the murderer is not killed, then universality remains broken, and the immorality – the sin – persists.
We can now derive a general moral rule: what you do unto others, others may do unto you. I will refer to this as the Rule of Universality.
Don't steal if you don't want to be stolen from. Don't hit if you don't want to be hit. Don't kill if you don't want to be killed.
It's simple. It's elegant. It's easy to understand. Even a child should be able to follow this moral rule, so there's no excuse for immoral behaviour. You must respect universality. You must not hold yourself to a different standard than everyone else. If you think it's okay for you to hit other people, you cannot complain when they hit back.
If you disagree with the Rule of Universality, then you must propose an alternative moral system. As you go through your thought process, you must keep in mind that simply stating that something is immoral is insufficient for a moral rule. If you state that X is wrong, you must also prescribe a punishment or remedy for X, and you must explain why your punishment or remedy does not contradict your moral system. The Rule of Universality elegantly explains why something is wrong, and prescribes a remedy that does not contradict the moral rule. Hitting is wrong if you don't allow others to hit you back. Therefore, the remedy is that those who hit may, themselves, be hit. The punishment/remedy does not contradict the moral rule.