Kamsa always saw himself as Krishna’s terminator.  “I have to kill Him, but I know that He cannot be killed. But still I will try my very best.” These were Kamsa’s thoughts. They were raw, strong and intense. In fact, their intensity attracted the minds of transcendentalists like Narada Muni. Narada Muni had once prayed, “Oh, how much I would like to love Krishna as much as the demons like Kamsa want to hate Him.” What he appreciated is the voltage generated by Kamsa’s emotions and NOT that he wanted to think like him and be inimical to Krishna. But why would someone hate God so much, and can one be successful in carrying that hatred to its ultimate conclusion to terminate God? Notionally yes, factually no!
How is it possible, notionally?
By denying His existence, at least verbally, propounding theories about how even after searching all over the universe He cannot be seen, by citing umpteen things which are going wrong in the material world and putting the blame squarely on God, and so on and so forth.
Kamsa was in a humble mood (rare for him) when he decided to take the reins of the chariot driving his newly married sister and brother-in-law to their residence. Alas, at that moment an unembodied voice warned Kamsa that the eighth child born of this union will be his own nemesis. From that day on, Kamsa was so conscious of Krishna that this constant thinking about Krishna would put the foundling faith of neophytes like me to utter shame. 
Krishna escaped from Kamsa’s palace and was sheltered in Gokula where not only did He survive numerous assassination attempts but all those would be assassins were dispatched to the netherworld by Krishna Himself. At least by now, some wisdom should have trickled into Kamsa’s skull about the futility of his scheme but no, he was determined to fight till the very end. King Kamsa had sent Akrura to Vrindavana to fetch the two young boys, Krishna and Balarama, to Mathura for a wrestling match. Under the guise of a wrestling tournament, which the villagers of Vrindavana were very fond of watching, Kamsa actually wanted to kill Krishna and if possible Balarama too. It is but natural to want to kill someone who wants to kill you. But this story about the rivalry between Kamsa and Krishna becomes an important philosophical issue when one understands that Krishna is the source of all creation, the Supreme Absolute Truth. In a very important paragraph in the Bhagavad-gita Krishna says, “He merely reciprocates.” This is very significant. Krishna does not merely say that He is good or bad or mixed or otherwise. He states that He is neither friendly nor does He have any animosity towards anyone. He merely reciprocates. This means that if I behave like Krishna’s enemy, only then does He behave like my enemy, not otherwise. The very moment Krishna along with Balarama entered the stadium, the entire crowd took notice of Him. But how they noticed Him is also very significant: The wrestlers (each one with a body like a giant whose physical appearance would put the modern WWF characters to flight) took Him to be like a thunderbolt. They were furious upon seeing Him. The simple men folk of Mathura polis had come to come to spend their time in some simple folk entertainment, and therefore they took Krishna as one of the best males. The ladies of Mathura, however, felt conjugally attracted upon seeing Krishna’s personal beauty. At the same time those who had accompanied Krishna from His village just laughed saying, “This is our own laddie, look how nice he looks.” Kamsa’s henchmen and lackeys (other minor kings or chieftains ) saw in Krishna the Divine chastiser or punisher. And, finally, what did Kamsa see? He saw The Terminator; he saw DEATH, his own death.
Now, if someone is so obsessed with killing Krishna and according to Krishna’s promise ( that no matter what happens, He will surely reciprocate ) Krishna has to accept him as His enemy. So after Krishna entered the stadium, He dragged Kamsa down from the high podium and straddling him, gave him blow after blow till Kamsa finally called it a day and left his body. Was this the end of the story? No. Anyone killed by Krishna has to get purified of all material contamination. Animosity or rebellion towards God is the ultimate contamination. Purified of all sins by Krishna’s blows, Kamsa ultimately attained a higher status in the spiritual world. The same purification is available for you and me if we simply chant the maha-mantra: Hare Krishna Hare Krishna Krishna Krishna Hare Hare/ Hare Rama Hare Rama Rama Rama Hare Hare, taste Krishna-prasada ( food lovingly offered to Krishna and sanctified by Him), or study the Bhagavad-gita. Following these methods, one can enjoy a much more pleasant and far less distressful alternative path to reach the spiritual realm, and not end up receiving blow upon blow from Krishna, which Kamsa felt as thunderbolts.