כי תעבד, God cursed the earth because Kayin’s occupation involved getting his sustenance from the earth. לא תסף תת כוחה לך, even though the earth had already been cursed ever since Adam had eaten from the tree of knowledge, now G;d added another curse on account of Kayin. He said that even the amount of yield that the earth had been in the habit of providing since being cursed previously, it would not be capable of producing for Kayin the murderer. It is entirely possible that this new curse was restricted to the results Kayin’s efforts would have, and does not mean that all other human beings, farmers, were made to suffer for Kayin’s act of fratricide. Possibly, it applied only to Kayin and his direct descendants until the time of Noach. Maybe it even applied also to the descendants of Sheth until Noach. It seems that with Noach’s birth, Lemech his father, had knowledge that the curse had run its course. We read in Bereshit Rabbah 22,10 that in the view of Rabbi Eleazar the word לך in our verse means that whereas the earth would not yield up its strength for Kayin, it would do so for others. On the other hand, Rabbi Yossi bar Chanina is on record that the curse applied both to Kayin and to others. It appears more likely that the word לך in our verse is indeed restrictive and means that other people farming the earth would experience better success than Kayin.1 There is no question, however, that the first curse earth experienced after Adam had eaten from the tree of knowledge was a permanent curse still in effect nowadays. The word בעבורך in 3,14 has a different connotation entirely from the word לך, meaning “on your account,” not “for you personally.”
נע ונד תהיה בארץ, this is yet another curse, one that most certainly applied only to Kayin personally, a curse which resulted in his not being able to feel at home permanently in any one place on earth. This was something psychological, forcing Kayin to adopt a nomadic lifestyle, trying his luck at growing things by moving to a new location every time he experienced crop failure. God did not kill him outright, something that every murderer deserves. The reason was that killing him now would be counterproductive as it would leave the earth without males other than Adam, and Adam could not be expected to sire more children after he had lost the two sons he had helped produce. We know, that even with Kayin surviving, the trauma of what happened prevented Adam from maintaining marital relations with his wife for 130 years. (5,3)
Although God had not killed Kayin, he was under sentence of exile in addition to the earth having been cursed as an instrument of providing his livelihood. The words נע ונד are as if the word נע had been repeated, stressing the seriousness of the curse. Our sages, in commenting on the line כל הורג קין שבעתים יוקם, “that anyone killing Kayin, he (Kayin) will be avenged sevenfold,” explain that seeing that Kayin personally had not been executed for his murder, people would reason that murder goes relatively unpunished. In order for people not to draw such a conclusion from what had not happened to Kayin, the curse of murdering even someone already as guilty as Kayin, had to be highlighted in such terms. (
Bereshit Rabbah 22,
12)
Another reason explaining why Kayin was not subjected to immediate execution is that Kayin, being the first murderer, had not known that the penalty for murder is execution. Subsequent generations knowing the penalty for murder, could not expect to be treated so leniently. It is even possible to consider Kayin’s killing Hevel as having been an inadvertent killing, seeing that he did not know what injury would prove fatal. He had never seen a person who had been killed, and when hitting Hevel he had not thought that he had injured him fatally. If that is so, the penalty of exile, i.e. נע ונד תהיה בארץ, “you will not be at home anywhere on earth,” is exactly the penalty devised by the Torah for people who killed inadvertently In Torah Shleymah on our chapter item #96 the word נע is understood to mean that Kayin will be forever on the move, whereas the word נד is understood to mean that he will not be able to sit down and relax but that his body will remain in constant motion, much like that of a drunken person. Once he had repented, one of these curses was removed from him, as we know from Genesis 4,15 וישב בארץ נוד. The Torah did not say, however, בארץ נוע, because he was not able to settle in one location for long.
1. Kayin’s failure as a farmer would demonstrate to the others that he, and not they, had indeed been punished on an ongoing basis for his having committed murder, hoping to benefit by it at the expense of his brother.