"The great wizards who were able to assume any form were engaged by the King of Bhoja, Kamsa, to kill Krsna, but in the course of His pastimes the Lord killed them as easily as a child breaks dolls.
Purport: The atheist Kamsa wanted to kill Krsna just after His birth. He failed to do so, but later on he got information that Krsna was living in Vrndavana at the house of Nanda Maharaja. He therefore engaged many wizards who could perform wonderful acts and assume any form they liked. All of them appeared before the child-Lord in various forms, like Agha, Baka, Putana, Sakata, Trnavarta, Dhenuka and Gardabha, and they tried to kill the Lord at every opportunity. But one after another, all of them were killed by the Lord as if He were only playing with dolls.
"Thereafter, His father, being afraid of Kamsa, brought Him to the cow pastures of Maharaja Nanda, and there He lived for eleven years like a covered flame with His elder brother, Baladeva.
Purport: There was no necessity of the Lord's being dispatched to the house of Nanda Maharaja out of fear of Kamsa's determination to kill Him as soon as He appeared. It is the business of the asuras to try to kill the Supreme Personality of Godhead or to prove by all means that there is no God or that Krsna is an ordinary human being and not God. Lord Krsna is not affected by such determination of men of Kamsa's class, but in order to play the role of a child He agreed to be carried by His father to the cow pastures of Nanda Maharaja because Vasudeva was afraid of Kamsa. Nanda Maharaja was due to receive Him as his child, and Yasodamayi was also to enjoy the childhood pastimes of the Lord, and therefore to fulfill everyone's desire, He was carried from Mathura to Vrndavana just after His appearance in the prison house of Kamsa. He lived there for eleven years and completed all His fascinating pastimes of childhood, boyhood and adolescence with His elder brother, Lord Baladeva, His first expansion. Vasudeva's thought of protecting Krsna from the wrath of Kamsa is part of a transcendental relationship. The Lord enjoys more when someone takes Him as his subordinate son who needs the protection of a father than He does when someone accepts Him as the Supreme Lord. He is the father of everyone, and He protects everyone, but when His devotee takes it for granted that the Lord is to be protected by the devotee's care, it is a transcendental joy for the Lord. Thus when Vasudeva, out of fear of Kamsa, carried Him to Vrndavana, the Lord enjoyed it; otherwise, He had no fear from Kamsa or anyone else."
"Sometimes when Krsna and His sakhas went to the pasturing ground, Kamsa would send a demon to kill Krsna. Therefore, almost every day there was a fight with some different kind of demon. After fighting with a demon, Krsna would feel fatigued, the hairs on His head would be scattered, and the sakhas would immediately come and try to relieve Him in different ways. Some friends would say, "My dear Visala, please take this fan of lotus leaves and fan Krsna so that He may feel some comfort. Varuthapa, you just brush the scattered hairs on Krsna's head which have fallen upon His face. Vrsabha, don't talk unnecessarily! Immediately massage Krsna's body. His arms have become tired from fighting and wrestling with that demon. Oh, just see how our friend Krsna has become tired!" These are some examples of the treatment given to Krsna by the sakhas."
Nectar of Devotion, Chapter 41
Bhaktivedanta Book Trust. Excerpted from text and purport of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.
Kamsa is Killed in Mathura