Every day, dozens of opportunities to benefit at the misfortune of others avail themselves to us, and so everyone now and again considers the question, "Hey, why the hell not?"
Indeed, why should an individual be moral; why should we not accept that this world is ours for the taking and anyone who gets in our way is simply a casualty of that situation? Why does the prohibition on taking advantage of others exist? Each of these questions asks basically the same question: why should I be moral; that is, what's in it for me?
An example would do nicely, but first, some background: In the game of racquetball, a rally begins with one player, X, serving to the front wall, and the receiver, Y, returning the ball to the front wall. Player X then hits the ball back to the front wall, as does Y, and so on until one of the players is unable to return the ball. When the players first line up for the serve, Player X is required to serve from the service box, a small area near the middle of the court, with his or her back to Player Y, who remains in back court to receive the ball.
It is well known that in tournament play coaches often tell athletes, upon receiving the first serve, to hit the ball as hard as possible directly at the back of their unsuspecting opponent. When a person is hit by a ball in racquetball, an "unintentional hinder" is usually called (this is especially the case in casual play) and the rally is played over—so this intentional "accident" will not affect Player Y's score if s/he were to do this to Player X.
But why do this in the first place? The answer is quite simple: racquetball is played on a small, enclosed court and balls can travel in excess of 100 miles per hour in professional tournaments. With that in mind, it is easy to see that a player cannot afford to be "ball shy." Once hit, most racquetball players will play more cautiously, giving their opponents more room to play (making sure to be out of the way each time he or she hits the ball!). This has the same chilling effect as when a batter is hit by a pitch in baseball, which usually results in the batter backing off the plate and giving more room for the pitcher to use (which is just what the pitcher wanted in the first place).
By bringing Player X's game down a notch, Player Y benefits immensely and seemingly does not incur any negative consequences.
Despite this fact, to take advantage of rules that allow players to unintentionally hit one another and intentionally strike an unsuspecting person in a non-contact sport is clearly immoral. Aside from the temporary pain it would cause him or her, there is also the danger that such an action could severely injure Player X, depending on where and how hard the ball is hit. Clearly, the desire to win a game is insufficient ethical justification for causing suffering on the part of another human being.
In response to this claim one might argue: "But who cares about Player X? S/he should be looking out for Player X, just as I should be looking out for my own interests." The argument does seem to have a point—it is clearly in your advantage to hit Player X and increase your chances to win the match. Why worry about his or her feelings when you should only be looking out for yourself? If dishonesty helps you win, then it is acceptable. As the character Dark Helmet said in Mel Brooks' 1987 movie Spaceballs, "Evil always wins because good is dumb."
There exists a Prisoner's Dilemma-esque a flaw in this logic, however. For if Player Y hits Player X and gets away with it, others will probably take notice and start doing the same (as they have done in real life). In fact, Player X might just turn around and hit Player Y the next time it is Player Y's turn to serve. If not, there are still many other racquetball players in the world who just learned a really cool way to psyche out their opponent and eventually someone will end up using the same trick against Player Y.
A religious thinker might respond by quoting the Golden Rule or something similar. "Hit not Player X with the racquetball unless ye wish to be hit with the racquetball thyself," s/he might read from some ancient tome of competitive sports theology. Whether the action is outlawed by a general rule or a specific commandment, the basic idea is "Don't do Q because God said not to do Q."
When challenged with a response such as, "Then you are a sucker," the religious thinker can simply point to what they believe to be their reward for following God's word, whether that be a dinner with L. Ron Hubbard, forty virgins, heaven, or whatever. It is for this prize the religious person toils; by putting off immediate satisfaction (e.g. winning the racquetball tournament) s/he is able to obtain a better prize in the future.
I find this argument, known to philosophers as Divine Command Theory, disappointing, as it accepts that anything is moral as long as God says so. For example, the commandment "Torture No Springs three days out of every week," if found in a reputable religious work, could soon become justification for thousands of religious persons to hurt the venerable author of this writeup.
Likewise, moral codes based on popular religions like Christianity must accept the subjugation of women, since, like torturing No Springs, it is explicitly condoned in the Bible.* It becomes obvious at this point that anything can be moral as long as God says so—but this goes against common sense, which dictates "murder is wrong because you just should not kill people," not "murder is wrong only because God said 'thou shalt not kill.'"
In this case, I would point to a rather utilitarian view of Social Contract Theory. Why should we be moral? Why should Person Y not gain an advantage over Person X by whacking him or her over the head with the ball? Because, simply put, Person Y does not want to live in a society where s/he is constantly getting hit in the back of the head with racquetballs. (Understandable.)
Knowing the Prisoner's Dilemma as s/he does, Person Y opts not to go down the slippery slope of creating a society in which nailing one another with little blue rubber things is morally acceptable.
To hit Player X is to take the first step towards a time in which it is acceptable for anyone to hit one's opponent in that way, thus creating a sport in which everyone is constantly looking over their shoulders and wondering when the ubiquitous intentional strike is coming. Unless new rules are introduced, the game will eventually become less fun (and more painful) for everyone, including Player Y. So in the end, it is in Player Y's best interest to simply return the ball to the front wall and try to win the match on his or her merit, rather than with the assistance of dirty tricks.
*See Leviticus 19:20-22, Deuteronomy 22:28-29 and 25:11-12, I Corinthians 11:3-15 and 14:34-35, Ephesians 5:22-23, Colossians 3:18, and I Timothy 2:9-14.