In the midst of the famous story of Kayin and Hevel the Torah offers us the very first lesson about the yetser hara (negative inclination); after Hashem ignores Kayin’s offering and turns to that of Hevel, Kayin reacts very badly. “Kayin was very angry and his countenance fell.” In response to this reaction, Hashem warns Kayin about the possible consequences of his reaction; “And Hashem said to Kayin, why are you angry, and why has your countenance fallen? Is it not true that if you do good, you will be forgiven? But if you do not do good, sin crouches at the opening; its longing is toward you, yet you will rule over it.”
The Kli Yakar explains this enigmatic verse with a Gemara in Brachos; the Gemara compares the yetser hara to a fly. He explains that a fly does not have enough power to open a person’s skin when there is no wound there, however when he finds an opening, he has enough power to strike and open the wound further. So too, the yetser hara does not have enough strength to expose a weakness of a person who has not faltered in his Avodas Hashem. However, when a person weakens he creates an opening through which the yetser hara can enter and cause the person to sin further. This is the explanation of the Gemara in Shabbos that says; “of the one who comes to sin it is opened for him.” The Kli Yakar explains that the opening referred to is that he has opened the door for the yetser hara to enter and widen the opening until the wound becomes impossible to heal.
The yetser hara stands at the opening, waiting for a person to falter, but if he does not then he will rule over the yetser hara because it does not have the strength to cause him to sin if he stands steadfast in his avodas Hashem. However, if a person does falter, even in a small way, then the yetser hara can enter the opening and it will be far more difficult to now overcome it’s temptations. Hashem was telling Kayin that he was on the brink of falling into the trap of the yetser hara by opening the door to him. Kayin did not heed Hashem’s advice, and instead he confronted Hevel, allowing his yetser hara to overcome him and lead him to commit the terrible sin of murder.
Based on this understanding the Kli Yakar suggests that a person who has foresight should strive not to give the yetser hara any opening through which it can enter and cause great damage. From here we learn the importance of consistency in one's avodas Hashem. It is well-known that when a person undertakes some kind of self-discipline, such as a diet, or giving up an addiction, it is essential that he strive to avoid faltering in any way. This is because of the Kil Yakar’s principle - once a person shows a sign of weakness, then he triggers a process of deterioration that is very hard to stop. It seems that people who have tried to give up something, and failed, in many instances did not make a conscious decision to renew their bad habit. Rather they had a moment of weakness where they made an exception to their discipline, however that one moment of weakness began the process whereby they fell back into the traps of that addiction.
Very often, people who commit terrible sins began with a very minor transgression. One Rabbi suggested that this was the case with the tragic cases of ‘observant’ butchers who descended to the point where they were selling non-kosher meat. How could someone fall to such a low level to the extent that he causes so many people to sin? He explained that they probably began with a very minor ‘short-cut’ that saved some money, and in time this slowly escalated until the yetser hara has completely taken over the person.
Not everyone will necessarily fall to this level when they falter, however there is still a great need for a person to try to remain consistent in his lifestyle and avoid even minor failings as much as possible. This applies in all areas of Avodas Hashem. In one’s own observance of mitzvos, small lapses can lead to bigger ones. It similarly applies with regard to one’s relationships - very often a relationship can deteriorate as a result of one thoughtless comment and after the damage is done it is far harder to rectify the situation. It is especially applicable in the area of learning - Chazal emphasize that a person must have set times for learning that are never missed; by consistently learning at a set time a person can overcome the yetser hara’s attempts to take him away from his learning. However, if he does not consistently adhere to his schedule then it is far easier for the yetser hara to drag him away from learning on a constant basis.
The story of Kayin teaches us about the importance of remaining steadfast in one’s Avodas Hashem as a prime way of overcoming the yetser hara and the terrible consequences that can occur if we do not. May we all be zocheh to defeat the yester hara and reach our full potential.