Because Krsna embraced Putana's body with great pleasure and sucked her breast, although she was a great witch, she attained the position of a mother in the transcendental world and thus achieved the highest perfection - SB 10.6.37-38
SB Canto 10.1 to 10.13
The Supreme Personality of Godhead, Kṛṣṇa, is always situated within the core of the heart of the pure devotee, and He is always offered prayers by such worshipable personalities as Lord Brahmā and Lord Śiva. Because Kṛṣṇa embraced Pūtanā's body with great pleasure and sucked her breast, although she was a great witch, she attained the position of a mother in the transcendental world and thus achieved the highest perfection. What then is to be said of the cows whose nipples Kṛṣṇa sucked with great pleasure and who offered their milk very jubilantly with affection exactly like that of a mother?
These verses explain how devotional service rendered to the Supreme Personality of Godhead, whether directly or indirectly, knowingly or unknowingly, becomes successful. Pūtanā was neither a devotee nor a nondevotee; she was actually a demoniac witch instructed by Kaṁsa to kill Kṛṣṇa. Nonetheless, in the beginning she assumed the form of a very beautiful woman and approached Kṛṣṇa exactly like an affectionate mother, so that mother Yaśodā and Rohiṇī did not doubt her sincerity. The Lord took all this into consideration, and thus she was automatically promoted to a position like that of mother Yaśodā. As explained by Viśvanātha Cakravartī Ṭhākura, there are various roles one may play in such a position. Pūtanā was immediately promoted to Vaikuṇṭhaloka, which is also sometimes described as Svarga. The Svarga mentioned in this verse is not the material heavenly planet, but the transcendental world. In Vaikuṇṭhaloka, Pūtanā attained the position of a nurse (dhātry-ucitām), as described by Uddhava. Pūtanā was elevated to the position of a nurse and maidservant in Goloka Vṛndāvana to assist mother Yaśodā.