Throughout the first and second books in the cantos, Sol Weintraub was urged to sacrifice his only child, Rachel, and give her willingly to the Shrike.
It was clearly presented as the ultimate act that will save billions of human life and save the whole universe.
My question is: why? The same effect exactly (from the Shrike point of view) could be achieved by taking her by force or just killing her on spot. In the second book it's made clear what is so important about Rachel and I see why eliminating her will indeed have major impact, but the point is why it matters whether she's being sacrificed by her father or just killed as infant?
I still did not read the third and fourth books so in case the answer lies there please just tell that and I will delete this question.