kim adya tasmin karaniyam asu me
yad artha-tantro na vihanti vikramam
striyah svasur gurumatya vadho ’yam
yasah sriyam hanty anukalam ayuh
kim—what; adya—now, immediately; tasmin—in this situation; karaniyam—is to be done; asu—without delay; me—my duty; yat—because; artha-tantrah—the Supreme Personality of Godhead, who is always determined to protect the sadhus and kill the asadhus; na—does not; vihanti—give up; vikramam—His prowess; striyah—of a woman; svasuh—of my sister; guru-matyah—especially when she is pregnant; vadhah ayam—the killing; yasah—fame; sriyam—opulence; hanti—will vanquish; anukalam—forever; ayuh—and the duration of life.
Kamsa thought: What is my duty now? The Supreme Lord, who knows His purpose [paritranaya sadhunam vinasaya ca duskrtam Bg. 4.8], will not give up His prowess. Devaki is a woman, she is my sister, and moreover she is now pregnant. If I kill her, my reputation, opulence and duration of life will certainly be vanquished.
According to Vedic principles, a woman, a brahmana, an old man, a child and a cow should never be killed. It appears that Kamsa, although a great enemy of the Supreme Personality of Godhead, was aware of the Vedic culture and conscious of the fact that the soul transmigrates from one body to another and that one suffers in the next life according to the karmas of this life. Therefore he was afraid of killing Devaki, since she was a woman, she was his sister, and she was pregnant. A ksatriya becomes famous by performing heroic acts. But what would be heroic about killing a woman who, while confined in his custody, was under his shelter? Therefore, he did not want to act drastically by killing Devaki. Kamsa’s enemy was within Devaki’s womb, but killing an enemy in such a nescient state would not be an exhibition of prowess. According to ksatriya rules, an enemy should be fought face to face and with proper weapons. Then if the enemy is killed, the victor becomes famous. Kamsa very conscientiously deliberated upon these facts and therefore refrained from killing Devaki, although he was completely confident that his enemy had already appeared within her womb.
Link to this page: https://prabhupadabooks.com/sb/10/2/21