jānan prāg viṣṇunā hatam
yadubhiḥ sa vyarudhyata
ātmānam — personally; iha — in this world; sañjātam — born again; jānan — understanding well; prāk — previously, before this birth; viṣṇunā — by Lord Viṣṇu; hatam — was killed; mahā-asuram — a great demon; kālanemim — by the name Kālanemi; yadubhiḥ — with the members of the Yadu dynasty; saḥ — he (Kaṁsa); vyarudhyata — acted inimically.
In his previous birth, Kaṁsa had been a great demon named Kālanemi and been killed by Viṣṇu. Upon learning this information from Nārada, Kaṁsa became envious of everyone connected with the Yadu dynasty.
Persons who are demons, enemies of the Supreme Personality of Godhead, are called asuras. As stated in Bhagavad-gītā, the asuras, because of their enmity toward the Supreme Personality of Godhead, take birth after birth in asura families and therefore glide down to the darkest hellish regions.