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.

With pitiable prayers, Renuka, the mother of Parasurama and wife of Jamadagni, begged for the life of her husband. But the sons of
Kartaviryarjuna, being devoid of the qualities of ksatriyas, were so cruel that despite her prayers they forcibly cut off his head and took it away.

Lamenting in grief for the death of her husband, the most chaste Renuka struck her own body with her hands and cried very loudly, "O Rama, my dear son Rama!"

Although the sons of Jamadagni, including Lord Parasurama, were a long distance from home, as soon as they heard Renuka loudly calling "O Rama, O my son," they hastily returned to the asrama, where they saw their father already killed.

Virtually bewildered by grief, anger, indignation, affliction and lamentation, the sons of Jamadagni cried, "O father, most religious, saintly
person, you have left us and gone to the heavenly planets !"

Thus lamenting, Lord Parasurama entrusted his father's dead body to his brothers and personally took up his ax, having decided to put an end to all the ksatriyas on the surface of the world.

O King, Lord Parasurama then went to Mahismati, which was already doomed by the sinful killing of a brahmana. In the midst of that city he made a mountain of heads, severed from the bodies of the sons of Kartaviryarjuna.

With the blood of the bodies of these sons, Lord Parasurama created a ghastly river, which brought great fear to the kings who had no respect for brahminical culture. Because the ksatriyas, the men of power in government, were performing sinful activities, Lord Parasurama, on the plea of retaliating for the murder of his father, rid all the ksatriyas from the face of the earth twenty-one times. Indeed, in the place known as Samanta-pancaka he created nine lakes filled with their blood.

(From SB Canto 9)