Kansa was the king of Mathura city and maternal uncle of Sri Krishna. According to legend, Janmashtami is celebrated as the birth of Bhagwan Krishna, an avatar of Lord Vishnu to slay the cruel king. Kansa was a self-proclaimed King of the city of Mathura (A City in Northern India). He overthrew his father, King Ugrasen and forcefully became the king. He was very wicked and evil in thinking. His subjects feared him, the cowherds of not only Mathura but also other neighboring villages like Gokul, Barsana, Nandgaon were the victims of his wickedness. They had no option but to surrender to his tyrannical ways.
Army of Demons
King Kansa had an army of demons who helped him spread fear and accomplish his evil tasks. Putana, Bakasura, Aghasura were his close aides.
Kansa loved his younger sister Devki and fulfilled all her wishes. He married her to Vasudev, one of his cousins and a high-ranking officer in the army with great pomp and show. However, during the wedding ceremony of his sister, Devki, the love for his sister turned to hatred when a heavenly prediction warned him, that the eighth child of Devki will be the annihilator of Kansa. On being assured of getting all the new born children of Devki and Vasudev, Kansa decided to imprison them under close supervision and let them live.
Killing of Infants
Kansa managed to kill all the new born of his sister Devki and brother-In-Law, Vasudev. His elder brother Balram, seventh child of the couple, was saved by being transferred to Rohini's womb. Similarly Lord Krishna, the eighth child, born to kill evil Kansa was saved after being miraculously shifted to Gokul on the night of his birth and replaced with a female child. However, Kansa got to know that his destroyer had been born and was being brought up by an unknown family in a neighboring village.
Ongoing Efforts to Kill the Child
Soon, after getting to know that the eighth child of his sister was born and safely growing in another village, Kansa ordered his soldiers to kill all the one-day old male infants in and around Mathura. He made quite a few unsuccessful attempts to kill the child.
Episode of Putana-The Demoness
King Kansa, sent one of his reliable servants to kill on the infants in the village by feeding them poisonous milk. She killed many infants but in the end she was killed by baby Krishna who sucked her life while being fed poisonous milk.
Killing of Demon Trinivarta
On hearing the death of Putana, King Kansa, sent his another servant- demon Trinivarta to kill Sri Krishna. Trinivarta took the form of a whirlwind, and carried the young Krishna away with itself, in the hope of throwing him away. Lord Krishna increased his weight which made the demon unable to move any further. After some time, the weight of Lord Krishna became too heavy for the demon to handle, so he fell on Ground and died.
End of Bakasura
At the instigation of Kansa, his friend Bakasura, changed his form to that of a crane and went to kill Young Krishna, He hit him with his beak but Lord caught hold of his beak and tore it apart, killing him instantly.
Slaying of King Kansa
King Kansa, tired of the killing of all his servants, invited both his nephews, Balram and Krishna for a wrestling match in Mathura. He made use of elephant Kuvalyapeeda to kill Krishna at the entrance of the arena, Krishna killed him followed by killings of Charuna and Toshalaka- chosen atheletes of Kansa. Krishna went to Kansa, took him by hair and threw him on the ground; finally killing him. The slaying Of Kansa ended his tyrannical rule and people rejoiced and celebrated his death.