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.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
आत्मानमिह सञ्जातं जानन् प्राग्विष्णुना हतम् । महासुरं कालनेमिं यदुभिः स व्यरुध्यत
ātmānamiha sañjātaṃ jānan prāgviṣṇunā hatam | mahāsuraṃ kālanemiṃ yadubhiḥ sa vyarudhyata
SB 10.1.68