The idea that was presented in the article posted in one of the two Prometheus threads was that the Engineers created life on this planet via an act of self sacrifice. Their race is totally comfortable with death. The idea of cheating death or not wanting to die disgusts them. When they bring the head to life it's horrified and destroys itself (lol?). When the live alien is asked for more life, it is disgusted and enraged and tries to kill everyone. It's not hostile until David (apparently) asks him to prevent raisin-man from dying. That's pretty much the only logical explanation that makes any semblance of sense. That ties into Space Jesus, in that the act of us killing one of them was the motivation for them to say "uh... wait a minute... we need a reboot down there..." The flood may have been a similar period where they said to themselves "yeah, we need to rethink the process a bit... maybe next time we should send one of us down there to supervise?"
However, We are talking about a period of time in the hundreds of millions of years, if not billions, from the point at which they created life on this planet to the point where we finally showed up at their Life Goo Storage And Distribution Facility. Since this is true, then everything about that previous theory is totally believable when compared to the notion that in the hundreds of millions of years since they started creating life, their civilization never apparently changed. They upgraded their spaceship design from a saucer to a wishbone. That's the result of 100 million years of technological development. Apparently. Their culture and philosophy apparently made no change in that time.
@ Trey: The cool thing about humans is that we are totally capable at looking at our own species and saying "wow... we are really fucked up..." and we do that quite a lot. Even when we acknowledge how fucked up we are, we are convinced that that acknowledgement makes us awesome.
EDIT: OK, I have to get this off my chest. The reply to Trey was originally just the first sentence. I wasn't sure if the point of the first sentence would be clear, so I added a second sentence that's basically the exact same sentence, but in reverse and a bit more obvious. I don't think even I would have gotten the first sentence's meaning, but I feel like I just broke that rule where you don't underestimate your audience. I feel dirty, but I dunno if I should. Should I feel dirty about that second sentence, or do you think it was necessary?