"Kamsa was an enemy of Krsna's. From the very beginning of Krsna's birth, he planned in so many ways to kill Him, and because he was always unsuccessful, he was always thinking of Krsna. Thus while working, while eating and while sleeping, he was always Krsna conscious in every respect, but that Krsna consciousness was not favorable, and therefore in spite of his always thinking of Krsna twenty-four hours a day, he was considered a demon, and Krsna at last killed him. Of course anyone who is killed by Krsna attains salvation immediately, but that is not the aim of the pure devotee. The pure devotee does not even want salvation. He does not want to be transferred even to the highest planet, Goloka Vrndavana. His only objective is to serve Krsna wherever he may be.
"Kamsa was made aware of his previous birth by the grace of Narada. He learned that in his previous birth he was a demon of the name Kalanemi and that he was killed by Visnu. Having taken his birth in the Bhoja family, he decided to become the deadly enemy of the Yadu dynasty; Krsna was going to take birth in that family, and Kamsa was very much afraid that he would be killed by Krsna, just as he was killed in his last birth.
He first of all imprisoned his father Ugrasena because he was the chief king among the Yadu, Bhoja, and Andhaka dynasties, and he also occupied the kingdom of Surasena, Vasudeva's father. He declared himself the king of all such places.
Krsna Book, Chapter 1
"...The great saint Narada descended from the heavenly planets to the forest of Mathura and sent his messenger to Kamsa. When the messenger approached Kamsa and informed him of Narada's arrival, Kamsa, the leader of the asuras, was very happy and immediately came out of his palace to receive Narada, who was as bright as the sun, as powerful as fire, and free from all tinges of sinful activities. Kamsa accepted Narada as his guest, offered him respectful obeisances and gave him a golden seat, brilliant like the sun. Narada was a friend of the King of heaven, and thus he told Kamsa, the son of Ugrasena, "My dear hero, you have satisfied me with a proper reception, and therefore I shall tell you something secret and confidential. While I was coming here from Nandakanana through the Caitraratha forest, I saw a great meeting of the demigods, who followed me to Sumeru Parvata. We traveled through many holy places, and finally we saw the holy Ganges. While Lord Brahma was consulting the other demigods at the top of Sumeru Hill, I was also present with my stringed instrument, the vina. I shall tell you confidentially that the meeting was held just to plan to kill the asuras, headed by you. You have a younger sister named Devaki, and it is a fact that her eighth son will kill you." (reference: Hari-vamsa, Visnu-parva 1.2-16)
No one can blame Naradaji for encouraging Kamsa to kill the sons of Devaki. The saint Narada is always a well-wisher for human society, and he wanted the Supreme personality of Godhead, Krsna, to descend to this world as soon as possible so that the society of demigods would be pleased and would see Kamsa and his friends killed by Krsna. Kamsa would also attain salvation from his nefarious activities, and this too would very much please the demigods and their followers. Srila Visvanatha Cakravarti Thakura remarks in this connection that Narada Muni sometimes did things that were beneficial to the demigods and the demons simultaneously. Sri Viraraghava Acarya, in his commentary, has included the following half-verse in this regard: asurah sarva evaita lokopadrava-karinah. Asuras are always disturbing elements for human society.
Bhagavad-Gita 10:1:69 Purport
".... Because Kamsa was not satisfied by Vasudeva's instructions, Vasudeva devised a plan. He offered to bring Kamsa all of Devaki's children so that Kamsa could kill them. Why then should Kamsa kill Devaki now? Kamsa was satisfied by this proposal. In due course of time, when Devaki gave birth to a child, Vasudeva brought the newborn baby to Kamsa, who, upon seeing Vasudeva's magnanimity, was struck with wonder. When Vasudeva gave Kamsa the child, Kamsa, showing some intelligence, said that since he was to be killed by the eighth child, why should he kill the first? Although Vasudeva did not trust him, Kamsa requested Vasudeva to take the child back. Later, however, after Narada approached Kamsa and disclosed to him that the demigods were appearing in the Yadu and Vrsni dynasties and conspiring to kill him, Kamsa decided to kill all the children born in these families, and he also decided that any child born from the womb of Devaki must be killed. Thus he arrested and imprisoned both Devaki and Vasudeva and killed six of their sons, one after another. Narada had also informed Kamsa that in his previous birth Kamsa was Kalanemi, a demon killed by Visnu. Consequently, Kamsa became a great enemy to all the descendants of the yadu-vamsa, the Yadu dynasty. He even arrested and imprisoned his own father, Ugrasena, for Kamsa wanted to enjoy the kingdom alone.
Srimad-Bhagavatam 10th Canto Summary
"Devaki, the mother of Krsna and sister of King Kamsa, was put into prison along with her husband, Vasudeva, because the envious King was afraid of being killed by Devaki's eighth son (Krsna). He killed all the sons of Devaki who were born before Krsna, but Krsna escaped the danger of child-slaughter because He was transferred to the house of Nanda Maharaja, Lord Krsna's foster father.
Srimad-Bhagavatam 1:8:23 Purport
"Some of the chief devotees, such as Akrura, stayed with Kamsa to satisfy him. This they did for various purposes. They all expected the Supreme Personality of Godhead to appear as the eighth child as soon as Devaki's other children were killed by Kamsa, and they were eagerly awaiting His appearance. By remaining in Kamsa's association, they would be able to see the Supreme Personality of Godhead take birth and display His childhood pastimes, and Akrura would later go to Vrndavana to bring Krsna and Balarama to Mathura. The word paryupasate is significant because it indicates that some devotees wanted to stay near Kamsa in order to see all these pastimes of the Lord. The six children killed by Kamsa had formerly been sons of Marici, but because of having been cursed by a brahmana, they were obliged to take birth as grandsons of Hiranyakasipu. Kamsa had taken birth as Kalanemi, and now he was obliged to kill his own sons. This was a mystery. As soon as the sons of Devaki were killed, they would return to their original place. The devotees wanted to see this also. Generally speaking, no one kills his own nephews, but Kamsa was so cruel that he did so without hesitation. Ananta, Sankarsana, belongs to the second catur-vyuha, or quadruple expansion. This is the opinion of experienced commentators.
Srimad-Bhagavatam 10:2:4-5 Purport
"King Kamsa not only occupied the kingdoms of the Yadu, Bhoja, and Andhaka dynasties and the kingdom of Surasena, but he also made alliances with all the other demoniac kings, as follows: the demon Pralamba, demon Baka, demon Canura, demon Trnavarta, demon Aghasura, demon Mustika, demon Arista, demon Dvivida, demon Putana, demon Kesi and demon Dhenuka. At that time, Jarasandha was the king of Magadha province (known at present as Bihar state). Thus by his diplomatic policy, Kamsa consolidated the most powerful kingdom of his time, under the protection of Jarasandha. He made further alliances with such kings as Banasura and Bhaumasura, until he was the strongest. Then he began to behave most inimically towards the Yadu dynasty into which Krsna was to take His birth.
Being harassed by Kamsa, the kings of the Yadu, Bhoja and Andhaka dynasties began to take shelter in different states such as the state of the Kurus, the state of the Pancalas, and the states known as Kekaya, Salva, Vidarbha, Nisadha, Videha and Kosala. Kamsa broke the solidarity of the Yadu kingdom, as well as the Bhoja and Andhaka. He made his position the most solid within the vast tract of land known at that time as Bharata-varsa.
When Kamsa killed the six babies of Devaki and Vasudeva one after another, many friends and relatives of Kamsa approached him and requested him to discontinue these heinous activities. But all of them became worshipers of Kamsa.
When Devaki became pregnant for the seventh time, a plenary expansion of Krsna known as Ananta appeared within her womb. Devaki was overwhelmed both with jubilation and lamentation. She was joyful, for she could understand that Lord Visnu had taken shelter within her womb, but at the same time she was sorry that as soon as her child would come out, Kamsa would kill Him. At that time, the Supreme Personality of Godhead, Krsna, being compassionate upon the fearful condition of the Yadus, due to atrocities committed by Kamsa, ordered the appearance of His Yogamaya, or His internal potency. Krsna is the Lord of the universe, but He is especially the Lord of the Yadu dynasty.
Krsna Book, Chapter 2
"This is a statement made by Sukadeva Gosvami, who was relating Srimad-Bhagavatam (10.4.46) to Maharaja Pariksit. This quotation concerns the attempted killing of Krsna's sister (Yogamaya), who appeared before Krsna's birth as the daughter of mother Yasoda. This daughter Yogamaya and Krsna were born simultaneously, and Vasudeva replaced Krsna by taking Yogamaya away. When she was brought to Mathura and Kamsa attempted to kill her, Yogamaya slipped out of his hands. She could not be killed. She then informed Kamsa about the birth of his enemy, Krsna, and being thus baffled, Kamsa consulted his associates, who were all demons. When this big conspiracy was taking place, this verse was spoken by Sukadeva Gosvami. He points out that a demon can lose everything because of his nefarious activities.
Caitanya-caritamrat, Madhyam Lila 15:270
"Purport: After Vasudeva bound himself with iron shackles as before, all the doors of the prison house closed by the influence of Yogamaya, who then began crying as a newborn child, This crying awakened the doorkeepers, who immediately informed Kamsa that a child had been born to Devaki. Upon hearing this news, Kamsa appeared with great force in the maternity room, and in spite of Devaki's pleas that the child be saved, the demon forcibly snatched the child from Devaki's hands and dashed the child against a rock. Unfortunately for Kamsa, however, the newborn child slipped away from his hands, rose above his head and appeared as the eight-armed form of Durga. Durga then told Kamsa, "The enemy you contemplate has taken birth somewhere else. Therefore your plan to persecute all the children will prove futile."
According to the prophecy, the eighth child of Devaki would kill Kamsa, and therefore when Kamsa saw that the eighth child was a female and heard that his so-called enemy had taken birth elsewhere, he was struck with wonder. He decided to release Devaki and Vasudeva, and he admitted before them the wrongness of his atrocities. Falling at the feet of Devaki and Vasudeva, he begged their pardon and tried to convince them that because the events that had taken place were destined to happen, they should not be unhappy for his having killed so many of their children. Devaki and Vasudeva, being naturally very pious, immediately excused Kamsa for his atrocities, and Kamsa, after seeing that his sister and brother-in-law were happy, returned to his home.
After the night passed, however, Kamsa called for his ministers and informed them of all that had happened. The ministers, who were all demons, advised Kamsa that because his enemy had already taken birth somewhere else, all the children born within the past ten days in the villages within Kamsa's kingdom should be killed. Although the demigods always feared Kamsa, they should not be treated leniently; since they were enemies, Kamsa should try his best to uproot their existence. The demoniac ministers further advised that Kamsa and the demons continue their enmity toward Visnu because Visnu is the original person among all the demigods. The brahmanas, the cows, the Vedas, austerity, truthfulness, control of the senses and mind, faithfulness and mercy are among the different parts of the body of Visnu, who is the origin of all the demigods, including Lord Brahma and Lord Siva. Therefore, the ministers advised, the demigods, the saintly persons, the cows and the brahmanas should be systematically persecuted. Strongly advised in this way by his friends, the demoniac ministers, Kamsa approved of their instructions and considered it beneficial to be envious of the brahmanas. Following Kamsa's orders, therefore, the demons began committing their atrocities all over Vrajabhumi.
Srimad-Bhagavatam 10:4 Summary
Bhaktivedanta Book Trust. Excerpted from text and purport of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.
Kamsa Sends Demons to Vrindaban to Kill Krsna
Kamsa is Killed in Mathura