SB 10.2.21

According to Vedic principles, a woman, a brāhmaṇa, an old man, a child and a cow should never be killed. It appears that Kaṁsa, 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 Devakī, since she was a woman, she was his sister, and she was pregnant. A kṣatriya 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 Devakī. Kaṁsa’s enemy was within Devakī’s womb, but killing an enemy in such a nescient state would not be an exhibition of prowess. According to kṣatriya rules, an enemy should be fought face to face and with proper weapons. Then if the enemy is killed, the victor becomes famous. Kaṁsa very conscientiously deliberated upon these facts and therefore refrained from killing Devakī, although he was completely confident that his enemy had already appeared within her womb.