In the midst of the story of Kain and Abel, the Torah offers us the very first lesson about the yester hara (negative inclination). After Hashem ignores Kain’s offering and turns to that of Abel, Kain reacts very badly. “Kain was very angry and his countenance fell.” In response to this reaction, Hashem warns Kain about the possible consequences of his reaction: “And Hashem said to Kain, ‘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[1].’ ”
The Kli Yakar clarifies this enigmatic verse with reference to the Talmud in Brachot, which compares the negative inclination to a fly[2]. He explains that a fly does not have enough power to open a person’s skin when there is no wound. However, when he finds an opening, he has enough power to strike and open the wound further. So, too, the negative inclination does not have enough strength to expose a weakness of a person who is steadfast in his avodat Hashem (service of G-d). However, when a person weakens, he creates an opening through which the negative inclination can enter and cause the person to sin further.
The negative inclination stands ready, waiting for a person to falter. If he remains steadfast, then that person will rule over it. This is because, as stated above, the negative inclination does not have the strength to cause a person to sin if that person stands his ground. However, if a person does falter, even in a small way, then the negative inclination can enter the opening. It will then be far more difficult to overcome temptations.
When He spoke to Kain about the “opening,” Hashem was warning him that he was on the brink of falling into the trap of the negative inclination by opening the door to him. Kain did not heed Hashem’s advice. Instead, he confronted Abel, allowing his negative inclination 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 negative inclination any opening through which it can enter and cause 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 return to their bad habit. Rather, in a moment of weakness, they relaxed their discipline. That one moment of weakness began the process whereby they fell back into the traps of that habit.
Very often, people who commit terrible acts began with very minor transgressions. Stories are told of thieves who began their 'careers' by taking candies from stores without permission. Most people will never fall to the level of committing terrible sins. Nevertheless, one must be vigilant in maintaining a consistent lifestyle, and promptly correct even the most minor lapses.
This applies in many areas of one's life. In particular, it is very relevant to our relationships with others. Sadly, even one small, thoughtless comment can have devastating effects on a relationship. After the damage is done, it can be very difficult to rectify the situation.
Consistency is also critical to Torah learning. The Rabbis emphasize that a person must have set times for learning that are never missed. Faithful adherence to one’s learning program can overcome the negative inclination’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 negative inclination to drag him away from learning on an increasing basis[3].
The story of Kain teaches us the importance of remaining steadfast in one’s service of G-d as a prime way of overcoming the negative inclination. May we all merit to defeat the negative inclination and reach our full potential.