“Screening [people]” in other way
…. If they had been apart,We should certainly have punished the Unbelievers among them with a grievous Punishment.
(Holy Qur`an, Surah AL-FATH, verse 25)
This is God’s destiny that [sometimes] disbelievers and hypocrites carry the deposits1of believers in their loins.
Hence Imam Ali (pbuh) didn’t kill their fathers [ancestors], in order for those deposits to be born [afterward].