Is the death of a son a punishment for the injustice of oppressors, and is the oppressed or the one asking [on their behalf] sinful for supplicating against them?

It is permissible to supplicate against an oppressor to the extent of his oppression. If the supplicant exceeds this, he becomes an oppressor himself. Forgiveness is closer to piety. What befalls the oppressor may be a punishment for him if Allah answers the supplication of one who was truly wronged by him.