Engineers didnt want to kill the Humans on Earth.

the fact the current inhabitant of the facility rips davids head off and uses it to kill weyland tends to lend to the fact these engineers want us dead. also, the different types of substances, black goo and sacrificial substance, also suggest we r on someones death list. the suggestive scenes of the movie tell us either another faction, probably engineers, attacked these engineers for whatever reason before they were able to launch theyr weapon on earth to wipe out humanity, or theyr experiment got loose, but the awakened engineers attitude tells us what theyr mission was. it adds weight to the idea that there is great conflict in engineer society. though what is unclear and open for debate is who the original occupants of the facility were. the mural suggest religious faction may have inhabited the facility before the current rebel faction took it over. not enough of the facility was explored to give concrete evidence who was there first. it is suggested, and its my theory, the facility is that of a militant rebel faction who disapprove of humanity for whatever reason. this is all open for debate as ridley is yet to gives us much of a backstory into engineer society between the time life was seeded on earth and the deaths of the engineers on lv223

I'm not sure if there is a consensus of invitation vs. warning either way.

it just seems to suggest conflict within the engineer society which I love the idea of. gives for great potential for the series/ and ridley, to show us great visual effects and insight into theyr past and culture. and obviously answer why we seem to have be marked for destruction

Hmmm... I like all the different ways everybody has used to form a legitimate debate as to the "why" in this entire storyline. I like to give things a new spin when permitted so ill mention , hold that thought.. I need to check to see if this morsel has been discovered in the Mu/th/ur forums yet, if not well then hold onto your hose.

Many people on here have used the phrases "bad for us" or "not good" and many similar terms. But if the Engineers truly created us, it's almost expected that the value of our lives is not high in their eyes. When David asked Charlie why he was made, David responded with "Because we could".

In Prometheus, David brings up the difficult question of how Charlie would feel if the Engineers responded the same way when asked the same question. 

If we're sticking with the idea that the Engineers truly did want to "destroy us", then it's possible that the whole point of our existence was to create beneficial hosts to create xenomorphs. Maybe they wanted a species that was close enough to produce the intended result, but with minor changes so that, 1- it couldn't be considered cruelty to their own species, and 2- to adjust their own genetic makeup so that humans, while being weaker and more controllable,  would produce a greater version of the xenomorph.

However, to avoid (what seems to have been) an outbreak of the xeno "virus", like on LV-223, it makes sense that they would create humanity as a docile, controllable pre-stage for the final product. Just like how people must feed and care for the mice they feed to their snake, Engineers put us on a safe world away from them where we could grow and prosper as a calm, delicate species. A species perfectly suited to be the host for the ultimate killing machine. 

(I hope my rant here made sense. I just re-watched the movie last night and I thought of this theory and wanted to share)
 

The Engineer sacrificed in the beginning of Prometheus is described by Scott as being a "perfect" being of their race, biologically speaking at least. It therefore seems possible that he is hinting towards the Engineers being bioengineered by the immortal (or very long living natural Elders). Just as David rebelled against his creators in Covenant, the Engineers rebelling against the Elders is a possibility. 

The whole thing is so wide open, its a problem i had when working on a Prometheus 2 draft... there are some aspects that you can have multiple theories that kind of step on each others toes and so its hard to narrow it down to a Definitive Theory.

There are flaws everywhere in some of the clues, flaws that make coming up with one theory, contradict another.

A simple example is that if we was created purely to be used as Xenomorph Hosts or to be used for Evolving into Black Goo Horrors, then its a pretty long winded Process that takes hundreds thousands of years (Spaights) or Hundreds of Millions (Lindeloffs) and why interact with us and teach us stuff, and the Star Maps... if we from day one was meant to be just Biological Fodder for the Creation of something else.

The only other way this would make sense if there are Thousands of Worlds at various times they Seed this way, it would be like a Wine Maker who makes Vintage Wines, he will start a batch, and this batch may be ready in 20 years, but then he starts another and another and another each 5 years apart... this way once his first 20 years is over, he will have a steady new batch every 5 years.

But then if they are creating Humanoids across hundreds or thousands of Worlds for just this Purpose... WHY? And to what end is the importance of creating Xenomorph type Organisms.?

The Deleted Elder Scene (which maybe we cant class as Canon) shows this Process was for a great cause and a benevolent act...

Ridley Scott mentioned the Space Jesus as being on the nose, but then his comments seem to be that they actually did not view us in any hostile way Thousands of Years ago, until we had done something and also proved to be a Failed Experiment.

The Reason for the Black Goo to be used, was because of how we turned out and became Rebellious and the risk is always there that we could one day Turn on them and Over throw them... they had a reason to be concerned with us, just as Mankind should had been with creating AI Androids... which alas, its proven the case that they have created something maybe they should not have because we see David has rebelled and became sentient.. if most of the Androids do the same, then we have a Skynet Terminator situation.

A Reason maybe the Engineers are cautious to us, and decided we needed to be destroyed is because maybe they in turn did this to their Creators/Hierarchy... and so its the same reason that Cronus Ate all his Children because he feared they would Rebel and want him Dead just as he did with his Father.

This is the Theme we was being shown... and so those LV-223 Engineers, Ridley called Fallen Angels indeed shows us they have rebelled themselves..

The Reason for the Black Goo, is maybe they Stole the Creation Goo to Create Mankind, maybe they just interfered in our development... but i think ultimately they got caught, got punished and the results of this Punishment created a Xenomorph related Ancestor

That these Engineers then held in high regard, they maybe saw this as a way they could give Birth to Life, that there Race was either never allowed but their Creators could. (If Engineers are clones and thus Biological Davids) Or if they had lost the ability to Procreate..

But now they have gained this in some way, and they worship this creation thats created from them and they chose to experiment on it and Perfect it and when they did they created the Mural, and they then decided to use its DNA in those Urns to Evolve Mankind.

into their New Perfect Creation.

The Engineers to a degree was doing what David is doing and maybe for similar reasons.

After thinking about Promethius I couldn't get away from the idea of two competing factions in the race of engineers. One was the creators and the other was the destroyers. The creators had a ship that was whole and round. The destroyers ship looked like a sickle cell blood cell. That to me spoke of genetic perfection vs genetic impurity and a kind of ideologically driven ethnic cleansing of a deficient bloodline called mankind. Notice Davids Arian look, yet ironically given the name of the great jewish hero. Could it be that the creators seeded life through out the universe but the destroyers found that to be somehow ethically or religiously blasphemous and unnatural? They sought to reverse the plaugue of humanity by loosing a vermin of ghastly efficiency on each world. Earth being the unlucky loser upon the discovery of the  base planet in Promethius. I would postulate the black goo is actually some form of programmed nanobot as opposed to actual genetic material. It uses the simple compounds to create what it is programmed to create. The creator sacrifice at the beginning seems to be taking upon himself a great honor as one perhaps chosen for his genetic perfection. The act is very ceremonious. The black goo he drinks did not create an alien but simply disassembled his body into the pollen of all life on earth so to speak, thus lending to the nanobot idea. The canisters seem to be filled with nano goo that uses any material it can to create an egglayer. The egg hatches and a xenomorph is born and feeds until mature. It then hunts and destoys humans as it was engineered to do. So at its heart its a story not unlike the history of man. There are creators and there are destroyers. The destroyers may be elegant and advanced but they are none the less diabolical and twisted. Its a Holocaust space opera.