One can see that in the same chapter, the Lord had resolved to incarnate on Earth as Sri Krishna, as the son of Devaki and Vasudeva.

Sri Suka also clearly explains that after giving his sister Devaki in marriage to Vasudeva, Kamsa himself rode the chariot seating the newly wed couple on the back.
The Lord wanted to give the best gift to the person who thus performed service to His parents-to-be! And the best gift that He could give is ‘Jeevan Mukti’ or liberation.

In Bhagavatam, it is said that one could attain God through untainted pure love, fear, enmity, kinship, friendship or devotion. Knowing the demonic nature of Kamsa, the Lord decided that it was impossible for Kamsa to attain the Lord through any Saatvic means and that He had to grant him the supreme state only through ‘bhaya’ or fear (as, by nature, Asuras fear for life).

The heavenly voice was a means of creating a sense of fear in Kamsa. This ‘prana bhaya’ or fear for life towards Krishna loomed large before Kamsa and due to the one-pointedness that he inculcated through fear for Lord Krishna, he could see Krishna anywhere and everywhere. This verily is the true state of Jivan Mukti!

Thus, for having done service to His parents, the Lord gave Kamsa the supreme state of bliss in a way that was most apt for him, and the ‘asarira’ or the heavenly voice was to give the fear to lead him to that state of Jivan Mukti.