As per Puranas, when Kamsa knew his sister, Devaki’s son will kill him, why did he not kill Devaki or her husband; and why did he still make them live together in the same cell?
Why did he not kill Devaki or Husband and yet made them live together in the same cell?
Kamsa did tried and was ready to kill Devaki at the time when the voice from the sky warned him about his death from the hands of Devakis eighth son . But Vasudeva pacified him with a promise that he will hand over all their sons to kamsa afterwards and hense Kaṁsa agreed to the logical arguments of Vasudeva, and, having full faith in Vasudeva’s words, he refrained from killing his sister. The answer is given in Shrimad Bhagvata Purana-Canto 10- Chapter 1.
After their marriage Vasudeva and devaki the sister of kamsa were departing towards their home. kamsa brother-in-law of Vasudeva driving their chariot who was accompanying them along the way. At that time a voice from the sky as a omen said to kamsa that eighth son from womb of Devaki shall slay you. kamsa who was already awicked sinful fellow thus being thus warned got ready to kill his own sister by holding of his sister’s hair with his left hand and took up his sword in right.
इत्युक्त: स खल: पापो भोजानां कुलपांसन: ।
भगिनीं हन्तुमारब्धं खड्गपाणि: कचेऽग्रहीत् ॥ SB 10.1. 35 ॥
Kamsa was a condemned personality in the Bhoja dynasty because he was envious and sinful. Therefore, upon hearing this omen from the sky, he caught hold of his sister’s hair with his left hand and took up his sword with his right hand to sever her head from her body.
But then Vasudeva the newly wedded husband of Devaki , intending to gain the favor of wicked kamsa started to pacify him.
श्लाघनीयगुण: शूरैर्भवान् भोजयशस्कर: ।
स कथं भगिनीं हन्यात् स्त्रियमुद्वाहपर्वणि ॥ SB 10.1.37 ॥
Vasudeva said: My dear brother-in-law Kaṁsa, you are the pride of your family, the Bhoja dynasty, and great heroes praise your qualities. How could such a qualified person as you kill a woman, your own sister, especially on the occasion of her marriage?
But since kamsa was Rakkshasa and was fiercely cruel he could be neither pacified nor terrified by the good instructions given by Vasudeva. He did not care about the results of sinful activities, either in this life or in the next and was determined to kill his sister Devaki.
Then in last resort Vasudeva in order to end the hostility promised to handover all their sons to Kamsa. Kaṁsa agreed to the logical arguments of Vasudeva, and, having full faith in Vasudeva’s words, he refrained from killing his sister.
न ह्यस्यास्ते भयं सौम्य यद् वैसाहाशरीरवाक् ।
पुत्रान् समर्पयिष्येऽस्या यतस्ते भयमुत्थितम् ॥ 54 ॥ स्वसुर्वधान्निववृते कंसस्तद्वाक्यसारवित् ।
वसुदेवोऽपि तं प्रीत: प्रशस्य प्राविशद् गृहम् ॥ SB 10.1.54 -55 ॥
Vasudeva said: O best of the sober, you have nothing to fear from your sister Devakī because of what you have heard from the unseen omen. The cause of death will be her sons. Therefore I promise that when she gives birth to the sons from whom your fear has arisen, I shall deliver them all unto your hands.
Śrīla Śukadeva Gosvāmī continued: Kaṁsa agreed to the logical arguments of Vasudeva, and, having full faith in Vasudeva’s words, he refrained from killing his sister. Vasudeva, being pleased with Kaṁsa, pacified him further and entered his own house.
So kamsa was not so foolish but since his danger was not Vasudeva nor Devaki nor even the first seven sons but the eighth son only . So he decided not to kill any of them but their sons only he then kept both of them in chains and killed every new-born child of them (seven of them). Kamsa kept full faith in Vasudeva’s words that they will give all their children to Kamsa did not kill them.