The myth of Kamsa is told in a series of nested myths that are an excellent study of karma as a process over many lifetimes. But its primary location is within the Krishna myth cycle, where Kamsa plays a role in Krishna’s nativity and his first great battle with evil (though in previous lives Krishna had already fought battles with demons of all sorts before the one with Kamsa).
Kamsa’s karmic chain began at the beginning of a new cosmic creation. Kamsa was a rebirth of Kâlanemi, a son of Virocana, an asura (demon) and the brother of Bali. After that rebirth as an evil human, his next rebirth was as Kâliya, a serpent king. Kâlanemi had had six sons, who were reborn from a curse by Brahmâ.
The six sons had a karmic chain that brought them into contact with Krishna. They had been reborn as sons of Kâlanemi because of what had happened in their previous lifetime. Brahmâ had created Marîci, whose life remains clouded in some mystery.
His greatness cannot be doubted, but some say that he was one of the Maruts (gods associated with Indra, thus gods of storms and battle). Others say that he was a Prajâpati (progenitor, or grandfather), or even one of the seven mahârishis, who were also the seven cosmic principles.
According to the Bhâ- gavata Purâna Marîci (light) and Ûrnâ married (in the Brahmânda Purâna his wife was Anasûyâ, “without spite”) and had six sons so powerful that they rivaled the creator. One day when they saw Brahmâ, they mocked him, saying that he was a father who had married his own daughter (Sarasvatî). Brahmâ cursed them to be born as demons.
And thus these six had rebirths in two important incarnations of Vishnu as sons of demons—first of Kâlanemi and then of Hiranyakas´ipu—and in one as humans, as sons of Vasudeva and Devakî. As sons of Hiranyakas´ipu the six brothers were pious and even received a boon from Brahmâ, that no one could kill them.
But Hiranyakas´ipu cursed them to sleep in Pâtâla (the underworld). After much pleading by his sons, he changed the curse so that they would be reborn as the first six sons of Devakî, so that their father in their previous life, Kâlanemi, who was going to be reborn as Kamsa, would be their uncle in the next lifetime and could kill them.
That would allow another rebirth, which was better than being stuck in Pâtâla. So the karmic chain of events brought Kâlanemi and the six sons of Marîci into a human rebirth. Kamsa was born as the son of Ugrasena, the king of Mathurâ. The meddling sage Nârada told Kamsa that he was the result of a rape of his mother by a gandharva named Dramila, and that that was why his mother hated him and cursed him to be killed by a member of his own family.
So Kamsa was not really surprised at the wedding of his sister Devakî when a voice from heaven said that his sister’s eighth son would kill him. Kamsa drew his sword and would have killed her, had not her husband Vasudeva pledged to give him each of the sons when they were born.
So began intermittent imprisonments of the couple and the killing of their first six sons, who were being reborn to work out karma going all the way back to their sin of mocking the creator. Kamsa personally bashed the brains out of each baby boy. Kamsa’s rule of Mathurâ became more and more evil.
The seventh child was a girl and an incarnation of Mâyâdevî. Even though the seventh birth was not a boy, Kamsa tried to smash her head against the ground. But she rose into the air and announced that Kamsa’s death was near. S´rî Krishna was born as the eighth son of Devakî and Vasudeva. Kamsa tried many ways to kill S´rî Krishna—all of which failed.
Finally, he was slain by Krishna when an attempted ambush backfired..