There are a number of reasons why a person should not sin: first, out of true love for God and neighbor which make it impossible for him to sin; second, out of fear of divine punishment; and third, because sin has a self-destructive, irrational nature. Some persons (e.g., Jerry Walls, drawing from Soren Kierkegaard) think that sin can play the role of a unifying principle of a person's personality ad infinitum. That is to say, a person may make a lifestyle out of sin and go on living that way indefinitely. This implies that sin does not lead intrinsically, of its own nature, to the destruction and sabotage of the human person. But I think the biblical teaching is otherwise.
Consider what John says about Cain and Abel:
We must not be like Cain who was from the evil one and murdered his brother. And why did he murder him? Because his own deeds were evil and his brother’s righteous (1 John 3.12).
Cain's first sin -- his initial evil deed -- mixed with his ego problems to produce a murderous potion. There is no reason Cain could not have responded differently to the rejection of his sacrifice by God. For example, he could have repented; he could have consulted Abel to see how he might sacrifice properly; he could have asked God for forgiveness. But to my mind it would seem he had an ego issue. He saw his brother righteous and accepted by God, he saw himself wicked and rejected by God, and refusing to accept this reality, he instead opted to murder the source of his self-esteem issues -- his own brother. Realizing what he has done, he fears for his own life (Gen 4.13-4), and only God's special intervention keeps this crime from being the end of him.
Sin therefore worked progressively in Cain's life to produce utter death; he goes on living only becaus God vows to protect his life through a threat of retribution. And we can see many other occasions throughout the scriptures in which sin works the ultimate undoing of the person who does not resist its temptations and inclinations. For example, look at the religious officials' response to Jesus' parable of the tenants (Mark 12.1-12). When Jesus tells the parable, it ends on a bad final note: the wicked tenants (who represented the religious officials of Israel) are destroyed by the vineyard owner. Mark tells us their reaction:
When they realized that he had told this parable against them, they wanted to arrest him, but they feared the crowd. So they left him and went away (Mark 12.12).
Though they are told that the murder of Christ will be their doom, yet in their sinfulness and hard hearts, they go on irrationally towards destruction. Christ is here foretelling the destruction of Jerusalem and the Temple in 70 A.D., a devastation which the Christians avoided by fleeing the city, in obedience to Jesus' warning, but the Jews and religious leaders were killed brutally. The sin within them compelled them further and further headlong into destruction.
These few examples tell me that sin has an irrational, self-destructive impulse. If we give in to it, we are liable to pay the penalty sooner or later -- and that penalty is death (Rom 6.23). This is something we see in real life examples as well: alcoholism turns into cirrhosis of the liver or cancer; addiction to pornography turns uncontrollable, making a man impotent in real sexual encounters; an unfriendly, bitter attitude leads to utter social isolation; and so on.