How could Prahlada watch his father be killed?

[Bhakti Vikasa Swami]

Prahlada Maharaja could have asked Nrsimhadeva, "My Lord, do not kill my
father." He could have asked, and immediately his request would have been
granted. But he did not do that. He was standing and seeing. But he knows
that "This killing of the body of my father is not killing. I shall save
him. I shall save him." That he did. When he was offered benediction, he did
not take anything. He said, "My Lord, what shall I do, all this material
benediction? I have seen in the case of my father, He was so strong that
even the demigods were frightened for his presence. His position was so
strong, and You finished it within a minute. So what is the benefit of
taking? No, no. Please do not request me." But although he did not ask
anything for his personal self, but he asked the Lord, prayed, that "One
thing I request." "What is that?" "My father was a great demon, and he was
against You. He has committed so many sinful life. So I know he'll have to
suffer for these things. I request You to excuse him." This is Vaisnava son.
"Oh, yes. Immediately." Immediately.

So he knew that "What is this of asking for this material...? My father is
not this body, but he has to suffer so many things for his sinful
activities. I shall save him in due course of time."
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From Srila Prabhupada's lecture on Srimad-Bhagavatam 1.8.47 - October 27,
1974, Mayapur