Śrīla Jīva Gosvāmī, in his notes on this verse, has mentioned how Nārada Muni gave Kaṁsa this information. This incident is described in the Hari-vaṁśa. Nārada Muni went to see Kaṁsa by providence, and Kaṁsa received him very well. Nārada, therefore, informed him that any one of the sons of Devakī might be Viṣṇu. Because Viṣṇu was to kill him, Kaṁsa should not spare any of Devakī's children, Nārada Muni advised. Nārada's intention was that Kaṁsa, by killing the children, would increase his sinful activities so that Kṛṣṇa would soon appear to kill him. Upon receiving the instructions of Nārada Muni, Kaṁsa killed all the children of Devakī one after another.
The word ajana-śaṅkayā indicates that Lord Viṣṇu never takes birth (ajana) and that He therefore appeared as Kṛṣṇa, taking birth just like a human being (mānuṣīṁ tanum āśritam). Kaṁsa attempted to kill all the babies born of Devakī and Vasudeva, although he knew that if Viṣṇu were born, He would not be killed. Actually it came to pass that when Viṣṇu appeared as Kṛṣṇa, Kaṁsa could not kill Him; rather, as foretold, it was He who killed Kaṁsa. One should know in truth how Kṛṣṇa, who takes His birth transcendentally, acts to kill the demons but is never killed. When one perfectly understands Kṛṣṇa in this way, through the medium of śāstra, one becomes immortal. As the Lord says in Bhagavad-gītā (BG 4.9):
- janma karma ca me divyam
- evaṁ yo vetti tattvataḥ
- tyaktvā dehaṁ punar janma
- naiti mām eti so 'rjuna
"One who knows the transcendental nature of My appearance and activities does not, upon leaving the body, take his birth again in this material world, but attains My eternal abode, O Arjuna."