One verse

Srimad Bhagavatam 9.16.11

9.16.11

दृष्ट्वाग्‍न्यागार आसीनमावेशितधियं मुनिम् । भगवत्युत्तमश्लोके जघ्नुस्ते पापनिश्चया: ॥ ११ ॥

drstvagny-agara asinam avesita-dhiyam munim bhagavaty uttamasloke jaghnus te papa-niscayah

drstva
by seeing
agni-agare
at the place where the fire sacrifice is performed
asinam
sitting
avesita
completely absorbed
dhiyam
by intelligence
munim
the great sage Jamadagni
bhagavati
unto the Supreme Personality of Godhead
uttama-sloke
who is praised by the best of selected prayers
jaghnuh
killed
te
the sons of Kartaviryarjuna
papa-niscayah
determined to commit a greatly sinful act, or the personified sins

The sons of Kartaviryarjuna were determined to commit sinful deeds. Therefore when they saw Jamadagni sitting by the side of the fire to perform yajna and meditating upon the Supreme Personality of Godhead, who is praised by the best of selected prayers, they took the opportunity to kill him.