They used sacrifices for the fear they kept people in.... Don't want your child to be sacrificed?... Better make a better offering to the church.... Don't want them sacrificed?... Better not piss off your local priest.... Christianity did the same thing and to a point still does whenever local law allows they call it witch burnings...

A twist of an older instinct, I believe.

When pursued by predators, like wolves attacking sheep, the prey flees; leaving the slowest behind. The wolves tear the sheep apart and the rest are spared.
Let's call the weaker sheep Jesus, whose lives were sacrificed so the many can live. Ever notice how offerings to gods in primitive cultures are often food?
Blood-sacrifices serves the same function as hamstringing someone in the tribe, sacrificing him to the terrible creatures that come out after dark. "Eat him, not me!"

Even the scape goat ritual that inspired the Jesus myth works exactly like that: You get a goat and expel it from the community (carrying sins, for some ridiculous magical reason) and the goat is eaten by predators we no longer need fear but do anyway because we are still primitive monkeys under our fine clothes.

big juju

It was a great tool for the shaman to get rid or blackmail the opponents. Just like PC today!

I've done that in my past religious life; NEVER again.

For me, sacrifice is wrong whether it be for a religion or a relationship. You should not have to prove your devoteness (yes I know that's not a word) to anything.