She was the sister of Bali, the Daitya “demon” who had conquered all the gods.
When Vāmana and approached Bali, she fell in love with him, “What a wonderful boy, I wish I was his mother!”
But when she saw how he manipulated Bali and stole everything from their family, her feelings turned to hatred, “I wish I could kill that boy!”
When Viṣṇu-Vāmana later appeared as Krishna, she became Pūtana, who tried to kill baby Krishna by playing the role of his mother – poisoning her breast milk.
When Krishna sucked out her prāṇa through her breasts, she died but did not attain the same type of liberation most enemies of Krishna attain. Most enemies relinquish their ego entirely and become homogenous within consciousness itself, brahman. But Pūtana instead developed a bhakti-ego and attained thereby a personal form in Krishna-līlā, as one of his wet-nurses.
Because her initial, primary feeling was one of affection for him, Krishna held on to this and disregarded her secondary, later feeling of anger over his arguable treatment of Bali. This shows that Krishna is not flawed by pride, anger, or unforgiveness.
One of the verses in Bhāgavatam about this is one of the three verses Vyāsa sent his students into the forest reciting, which succeeded in bringing Śukadeva Goswāmī back to learn the whole Bhāgavatam from Vyāsa.