There are seven kinds of mothers according to Vedic injunction: the real mother, the wife of a teacher or spiritual master, the wife of a king, the wife of a brahmana, the cow, the nurse and the mother earth. Because Putana came to take Krsna on her lap and offer her breast's milk to be sucked by Him, she was accepted by Krsna as one of His mothers. As Yasoda was given liberation from the material world, so Putana was also given liberation. When the baby Krsna closed His eyes, Putana took Him on her lap. She did not know that she was holding death personified. If a person mistakes a snake for a rope, he dies. Similarly, Putana killed so many babies before meeting Krsna, but now she was accepting the snake that would kill her immediately.- Krsna Book Ch-6 A.C. Bhaktivedanta Swami
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More to think about.
When Putana was called for by King Kamsa , he mentioned that he had a job for her. She had already been in the service of Kamsa for some time and was feeling very morose and ashamed of her sinful activities. When she was instructed to go to the house of Nanda and Yasoda to kill their child she refused. She was threatened again and again by Kamsa and finally submitted to his evil wishes. She smeared her breast with Arsenic and went to Gokul. When seeing baby Krsna for the first time, her maternal instincts overwhelmed her and milk began to flow profusely. She became so emotional that she forgot the instruction of Kamsa. Her only desire at that moment was to offer herself to Krsna. When Krsna was on her lap she thought to herself "my darling Krsna take from me what ever you like". She was completely surrendered to giving pleasure to Baby Krsna. The very next moment her life air passed. -AJD