When Jamadagni's wife, Reṇukā, went to bring water from the Ganges and saw the King of the Gandharvas enjoying the company of Apsarās, she was captivated, and she slightly desired to associate with him. Because of this sinful desire, she was punished by her husband. Paraśurāma killed his mother and brothers, but later, by dint of the austerities of Jamadagni, they were revived. The sons of Kārtavīryārjuna, however, remembering the death of their father, wanted to take revenge against Lord Paraśurāma, and therefore when Paraśurāma was absent from the āśrama, they killed Jamadagni, who was meditating on the Supreme Personality of Godhead. When Paraśurāma returned to the āśrama and saw his father killed, he was very sorry, and after asking his brothers to take care of the dead body, he went out with determination to kill all the kṣatriyas on the surface of the world. Taking up his axe, he went to Māhiṣmatī-pura, the capital of Kārtavīryārjuna, and killed all of Kārtavīryārjuna's sons, whose blood became a great river.
Renuka - wife of Jamadagni
SB Canto 1
Paraśurāma, or Reṇukāsuta: He is the son of Maharṣi Jamadagni and Śrīmatī Reṇukā. Thus he is also known as Reṇukāsuta. He is one of the powerful incarnations of God, and he killed the kṣatriya community as a whole twenty-one times. With the blood of the kṣatriyas he pleased the souls of his forefathers. Later on he underwent severe penances at the Mahendra Parvata. After taking the whole earth from the kṣatriyas, he gave it in charity to Kaśyapa Muni. Paraśurāma instructed the Dhanur-veda, or the science of fighting, to Droṇācārya because he happened to be a brāhmaṇa. He was present during the coronation of Mahārāja Yudhiṣṭhira, and he celebrated the function along with other great ṛṣis.
SB Canto 9
Satyavatī later became the sacred river Kauśikī to purify the entire world, and her son, Jamadagni, married Reṇukā, the daughter of Reṇu. By the semen of Jamadagni, many sons, headed by Vasumān, were born from the womb of Reṇukā. The youngest of them was named Rāma, or Paraśurāma.
Once when Reṇukā, the wife of Jamadagni, went to the bank of the Ganges to get water, she saw the King of the Gandharvas, decorated with a garland of lotuses and sporting in the Ganges with celestial women (Apsarās).
Later, understanding that the time for offering the sacrifice had passed, Reṇukā feared a curse from her husband. Therefore when she returned she simply put the waterpot before him and stood there with folded hands.
My dear King Parīkṣit, the sons of Kārtavīryārjuna, who were defeated by the superior strength of Paraśurāma, never achieved happiness, for they always remembered the killing of their father.
Jamadagni was certainly very powerful due to his austerities, but because of a slight offense by his poor wife, Reṇukā, he ordered that she be killed. This certainly was a sinful act, and therefore Jamadagni was killed by the sons of Kārtavīryārjuna, as described herein. Lord Paraśurāma was also infected by sin because of killing Kārtavīryārjuna, although this was not very offensive. Therefore, whether one be Kārtavīryārjuna, Lord Paraśurāma, Jamadagni or whoever one may be, one must act very cautiously and sagaciously; otherwise one must suffer the results of sinful activities. This is the lesson we receive from Vedic literature.
With pitiable prayers, Reṇukā, the mother of Paraśurāma and wife of Jamadagni, begged for the life of her husband. But the sons of Kārtavīryārjuna, being devoid of the qualities of kṣatriyas, 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 Reṇukā struck her own body with her hands and cried very loudly, "O Rāma, my dear son Rāma!"
Although the sons of Jamadagni, including Lord Paraśurāma, were a long distance from home, as soon as they heard Reṇukā loudly calling "O Rāma, O my son," they hastily returned to the āśrama, where they saw their father already killed.
“"Being ordered by his father, Paraśurāma killed his mother, Reṇukā, just as if she were an enemy. When Lakṣmaṇa, the younger brother of Lord Rāmacandra, heard of this, He immediately engaged Himself in the service of His elder brother and accepted His orders. The order of the spiritual master must be obeyed without consideration."
This is a quotation from the Raghu-vaṁśa (14.46). Lord Rāmacandra's statement to Sītā given below is from the Rāmāyaṇa (Ayodhyā-kāṇḍa 22.9).