In matsya purana, Indra killed many of Diti's children.
Why Indra killed many of Diti's children and why Indra death is calamity although lord Brahma don't mind and and agreed that Indra will killed by Vajranga's son?
"When the thousand years were over, Vajranga returned. He was shocked to learn of the tortures that his wife had been subjected to by Indra. He began to meditate again. This time, he wished for a son who would kill Indra. Brahma appeared and granted Vajranga the desired boon."