As far as the Sacrificial Scene in Prometheus.... it depends how they are evolving the importance of this scene and where it took place.
It was shot on Earth, but so was every other Sci-Fi movie ever made... only the Apollo missions was shot on the Moon or maybe lol
The plan in Prometheus was to show us that the Engineers used this Sacrificial route to seed life on Worlds to start the building blocks of life or the catalyst to evolve basic life.
And so when Ridley Scott said it did not have to be Earth, means that in reality the Engineers may have performed the same Ritual Sacrifices on other worlds as well as Earth too.
Now Alien Covenant has arrived, maybe the Sacrificial Scene could have been the first attempt at doing this... and on Paradise we simply cant be sure.
I have a feeling... and mentioned this in my Female's being the downfall of Engineers and Mankind thread.... that maybe Females and loss of females could be the reason they turn to the Sacrificial route?
Ridley did say they could create life in other ways... which could mean cloning or maybe sexual procreation... but we saw no Female Engineers, the Engineer seemed very interested in Shaw and well if we linked the Engineers with Fallen Angels and Ridleys comments about how in Paradise Lost the Fallen Angels had the most fun... and how you had one guy who would go to all the parties and get all the girls... while one guy stay at home and be boring.... This ties in with Biblical Genesis 6:4 about the Giants when Sons of God (Angels) slept with the Daughters of Man.
We see they did have some female Engineer concepts for Prometheus where the females where flat chested....
Alien Covenant those beings on Paradise we see females, and they have breasts lol..... We maybe have to look at this scene being a Ancient Punishment scene...
Where these beings had done something against their creators, and as punishment they had tried to destroy them by wiping out the female Engineers..... A gift taken from them by their Gods because they would Lust for and care for the Females more than their own creators.
This goes down to Bible too... if we study the Bible and Paradise Lost... had God not created Woman (Eve) for Adam but a Male companion or none at all..... its likely Adam would not have been tempted to Sin...... Unless Satan took form of a Female to tempt him.... and so Lust is a common theme of Sin, not only for Man but those Fallen Angels too.... its a theme followed in many cultures included Greek Mythos....
And even the Tale of Das Rheingold of which "enter the gods of valhalla" is part of... which one rumor mentioned in a Prologue Weyland had activated David and then began to talk about creation etc and David then went to a Piano and played "enter the gods of valhalla"
Which i think the trailer and shot still of David and the Piano all but 100% confirms this rumor to be true..
So maybe and sorry for going off Topic.... but the Sacrifice was needed because these Engineers had the Gift of Female Companionship taken away from them....
But alas the so called 4Chan leaks propose the Engineer Scene is as the result of David for no reason at all... and then for no reason after the Juggernaught crashed LOL