Hadley's HopeJune 10, 2012I think it was to deliberately show us that these creatures (Who we've assumed to be the engineers, but Is supect to be a caste of cloned worker drones) are brainwashed into worship as a form of keeping them in their place. Instead of what Christians use, an image of torture, not to threaten, but show salvation.. (and a 'we owe a debt for that' type of slave mentality)
These guys are given a reminder of the destructive power of their masters, who I believe also created the Xenomorph (which you have to admit seems to be a tailor made highly adaptable killing machine, surviving almost anywhere, and having acid blood which discourages people simply shooting them or stabbing them at close range)
I posted a [url=http://www.prometheus-movie.com/community/forums/topic/7640]theory[/url] that the ones trying to kill us are a different group to the ones who seeded the earth, and they only found out about us, (or got jealous, offended or threatened by our existence) much more recently... although why they picked a timeline around the supposed birth of christ might be just to mess with our heads or play into the "Jesus was an astronaut" myth.