This is a secret of the divine world! Souls you have heard of killed by God himself like Ravan, Shishupal, Pootna were actually saints. As Ravan, Shishupal, Pootna – they are just playing a Leela to be standing against God. In fact, their love for God is much more to have taken such a difficult seva.
Pootna was daughter of King Bali in previous lifetime. When Krishna took avatar as Baman Bhagwan and went to Lord Bali, she had a desire to nurture him as her child. God granted her boon that when he comes to Earth as Krishna, he will fulfill this desire. She then appeared as Pootna, fed Lord Krishna in Leela and went to His divine abode after being killed.