rser vinirgame kamso
yadun matva suran iti
visnum ca sva-vadham prati
devakim vasudevam ca
nigrhya nigadair grhe
jatam jatam ahan putram
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
After the departure of the great saint Narada, Kamsa thought that all the members of the Yadu dynasty were demigods and that any of the children born from the womb of Devaki might be Visnu. Fearing his death, Kamsa arrested Vasudeva and Devaki and chained them with iron shackles. Suspecting each of the children to be Visnu, Kamsa killed them one after another because of the prophecy that Visnu would kill him.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Srila Jiva Gosvami, in his notes on this verse, has mentioned how Narada Muni gave Kamsa this information. This incident is described in the Hari-vamsa. Narada Muni went to see Kamsa by providence, and Kamsa received him very well. Narada, therefore, informed him that any one of the sons of Devaki might be Visnu. Because Visnu was to kill him, Kamsa should not spare any of Devaki’s children, Narada Muni advised. Narada’s intention was that Kamsa, by killing the children, would increase his sinful activities so that Krsna would soon appear to kill him. Upon receiving the instructions of Narada Muni, Kamsa killed all the children of Devaki one after another.
The word ajana-sankaya indicates that Lord Visnu never takes birth (ajana) and that He therefore appeared as Krsna, taking birth just like a human being (manusim tanum asritam). Kamsa attempted to kill all the babies born of Devaki and Vasudeva, although he knew that if Visnu were born, He would not be killed. Actually it came to pass that when Visnu appeared as Krsna, Kamsa could not kill Him; rather, as foretold, it was He who killed Kamsa. One should know in truth how Krsna, who takes His birth transcendentally, acts to kill the demons but is never killed. When one perfectly understands Krsna in this way, through the medium of sastra, one becomes immortal. As the Lord says in Bhagavad-gita (4.9):
janma karma ca me divyam
evam yo vetti tattvatah
tyaktva deham punar janma
naiti mam 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.”