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.Śrīmad-Bhāgavatam 10.1.68
ātmānam iha sañjātaṁ
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.
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.