What was the motivation behind the Engineers actions in Prometheus?

The Engineers are planet builders who seed planets with the gift of life. It's shown in the start of the film that the Engineers visit Earth repeatedly, as their figure pointing to a constellation of stars is found throughout history.

When the film starts, an Engineer drinks the dark fluid, that starts this chain reaction of life. This fluid is so powerful, that it can turn a dead planet into a living Earth.

When the crew of the Prometheus lands on the alien planet, and they enter the tomb their foot steps uncover worms that are preserved in the tomb. They are still alive and crawling. The tomb cylinders start to leak the black fluid and the worms mix with the fluid. These worms evolve into the snake like creatures that later attack the two crew members stuck in the tomb.

Later, only the Prometheus captain refers to the black cylinders as bombs or weapons. When Elizabeth is talking to David she asks why the Engineers would want to destroy Earth, David replies "sometimes before you can create you must destroy".

I don't think the Engineers were going to destroy Earth as much as "restart" it. It's clear they took an active role in making our planet, but they had some kind of agenda that isn't defined in the film.

This is why, at the end of the film Elizabeth flies to their home world. To find out why the Engineers had decided Earth was a failure.

EDIT:

I answered this question two years ago, but since then a lot has been revealed about the plot for Prometheus by Ridley Scott. The following comes from various sources that I've read.

• At the start of the film. Illustrations of figures pointing to the stars implies this was a message from the Engineers. Ridley stated in an interview that there is nothing in the film that indicates these figures were Engineers, and the pointing could have been a warning not an invitation.

• Ridley says the early planet shown at the start where an Engineer sacrifices himself was not Earth, nor does it indicate if it was a historic reference or present day.

• Ridley Scott has said in an interview that the reason the Engineers wanted to destroy Earth (specifically mankind) was because Jesus Christ was an Engineer. The Engineers made man, man went bad so they sent an Engineer to try and fix the problem. We crucified the Engineer and apparently they didn't like this.

There are many other controversial topics related to the film, but I decided to only reference the things that came directly from Ridley.

Apparently, it's because of what happened (what humans did) to Jesus. Yes. That Jesus. Apparently he's an Engineer-Space-Jesus.

Excerpts from Prometheus Unbound: What The Movie Was Actually About [cavalorn.livejournal.com/584135.html] (emphasis, mine):

An astonishing excerpt from the Movies.com interview with Ridley Scott:

Movies.com: We had heard it was scripted that the Engineers were targeting our planet for destruction because we had crucified one of their representatives, and that Jesus Christ might have been an alien. Was that ever considered?

Ridley Scott: We definitely did, and then we thought it was a little too on the nose. But if you look at it as an “our children are misbehaving down there” scenario, there are moments where it looks like we’ve gone out of control, running around with armor and skirts, which of course would be the Roman Empire. And they were given a long run. A thousand years before their disintegration actually started to happen. And you can say, "Let's send down one more of our emissaries to see if he can stop it." Guess what? They crucified him.

Yeah. The reason the Engineers don't like us any more is that they made us a Space Jesus, and we broke him. Reader, that's not me pulling wild ideas out of my arse. That's RIDLEY SCOTT.

...

So how did our (in the context of the film) terrible murderous act of crucifixion end up wiping out all but one of the Engineers back on LV-223? Presumably through the black slime, which evidently models its behaviour on the user's mental state. Create unselfishly, accepting self-destruction as the cost, and the black stuff engenders fertile life. But expose the potent black slimy stuff to the thoughts and emotions of flawed humanity, and 'the sleep of reason produces monsters'. We never see the threat that the Engineers were fleeing from, we never see them killed other than accidentally (decapitation by door), and we see no remaining trace of whatever killed them. Either it left a long time ago, or it reverted to inert black slime, waiting for a human mind to reactivate it.

The black slime reacts to the nature and intent of the being that wields it, and the humans in the film didn't even know that they WERE wielding it. That's why it remained completely inert in David's presence, and why he needed a human proxy in order to use the stuff to create anything. The black goo could read no emotion or intent from him, because he was an android.

Shaw's comment when the urn chamber is entered - 'we've changed the atmosphere in the room' - is deceptively informative. The psychic atmosphere has changed, because humans - tainted, Space Jesus-killing humans - are present. The slime begins to engender new life, drawing not from a self-sacrificing Engineer but from human hunger for knowledge, for more life, for more everything. Little wonder, then, that it takes serpent-like form. The symbolism of a corrupting serpent, turning men into beasts, is pretty unmistakeable.

Refusal to accept death is anathema to the Engineers. Right from the first scene, we learned their code of willing self-sacrifice in accord with a greater purpose. When the severed Engineer head is temporarily brought back to life, its expression registers horror and disgust. Cinemagoers are confused when the head explodes, because it's not clear why it should have done so. Perhaps the Engineer wanted to die again, to undo the tainted human agenda of new life without sacrifice.

After seeing the movie, this question ate at me and I kept coming back to the same idea. Ridley Scott answered this question and confirmed what I suspected.

Whatever happened on LV-223, happened approximately 2000 years ago. It's clear that the Engineers were loading up their ships with cannisters (oddly shaped like 20th century bombs don't you think?) and plotting a course for earth.

It's clear the Engineers visited earth many times after the creation and interacted with humans. So at one point they were quite happy with us. But something changed.

What happened on Earth 2000 years ago that would anger our creator to the point that they decided enough was enough?

Well .. according to one of our largest religions, we killed a man who did nothing more than preach peace and love.

Every religion has the creation story and the story of some great event that nearly destroyed mankind. At some point, the creator becomes angry and starts over.

In Prometheus, "God" was pissed! Period. A messenger was sent down to warn us that we were on the wrong the path. Instead of heeding that warning, we crucified the messenger.

"God" had enough and decided to wipe out His creation. Only this time, the weapon intended to wipe us out turned on "God" and destroyed all of His kind first.

The movie is fairly clear that the engineers were responsible for the seeding of human life on earth (and probably all life unless the film makers were careless in how they described DNA similarities). But there is very little evidence given directly as to why they did this.

We also know the engineers must have interacted with early mankind or the archaeological traces with the star map would not have been created. But we don't have any evidence about what they did to early man.

One of the other threads in the movie is the somewhat naive desire in some characters to "meet their maker". Peter Weyland seeks their knowledge because it might preserve his fading life. Elisabeth Shaw seeks them out of some primal urge to understand where she came from (perhaps compensating for the early deaths of her parents). She ends with a strong desire to know whey they "rejected" humanity (almost as if she is asking "what did we do wrong?").

Everything shown about the engineers in the movie itself seems designed to contradict these naive views about them being benign "creators". The best theory about their work on the planet was that it was a military experiment, deliberately isolated from engineerdom because it was dangerous (I think this was the captain's idea). The storage areas for the "eggs" certainly suggest that they were being made and stored by the engineers. The stored videos in the pyramid complex certainly look like something went wrong.

So why the connection with earth? Given the above I think the engineers were simply creating species to use in their experiments. We were just sophisticated lab rats and they felt no more empathy with us then we would with rats killed in medical experiments. The woken engineer just wanted to destroy the people waking him and his urgency to act after waking might be a product of wanting to destroy humanity before it gets out of hand (clearly the original plan to test the weapons would have been on a pre-space age planet where the experiments could not escape).

So, in conclusion, it wasn't that they wanted to destroy humanity: they wanted to use us as guinea-pigs in a bio-weapon experiment.

The only mystery is why they interacted with ancient man (though they could just have been monitoring the experiment's progress).

I think that there are two groups of engineers. There was a split in their thinking that lead to war between them. One group of engineers wanted to create a race much like themselves, but others among them rebelled against the thought of creating others like themselves and in secret, created the xenomorphs as a weapon to use against the other element of engineers that may have been protecting humans. I think the war was devastating and very few survived it, so when the one was awakened he knew that his side must have failed in winning the war.

Notice that the surviving engineer did not bother to try and communicate with his homeworld, which leads me to believe that they all must be dead too or they think he is. He figured this out because it was humans that found him and not his own kind. Then there is also the fact that there are other working ships on the planet, no one leaves working interstellar hardware laying around on a military installation unless they are all dead. It will be interesting to see if Elizabeth finds them, but I think she will only find a dead homeworld.

There is also the other possiblity that the faction that may have been protecting us killed off the creators of the xenomorphs and left the planet for dead, but I don't think that if they were still alive they would have left such a dangerous weapon on the moon and planet unchecked. They did, however, leave a warning so they must have had some conscience. Hopfully we get to find out in a sequal.

My guess as to what happened:

The earth that they had populated in the beginning was another form of biological experiment to see how lifeforms feed off of each other, however, one life form (human), extended its reach beyond earth and into space.

Basically, the human bioweapon became smarter and posed a major threat to the existence of those that created the human bioweapon in the first place. Hence, when the engineer wakes up, he makes no attempt whatsoever to communicate with the humans and robot android, but realizes that the human bioweapon has extended its reach beyond earth, and cannot be contained.

Therefore, he kills everyone within his vicinity and starts heading for earth to destroy it completely using either the black liquid or other form of weapons.

In my opinion he kills them because human behavior imitates that of a virus and is a deadly weapon of warfare against other life forms.

Watching this video made me think of Prometheus. http://www.youtube.com/watch?v=9n-tTst-vyQ&feature=youtube_gdata_player I think the engineers were a type 1 civilization or were attempting to get to a type 2 civilization, they clearly had a mastery and understanding of the galaxy. My guess is she visits the planet and it is a wasteland, which also explains why the engineer/warrior did not try to communicate with the homeland prior to its attempt to destroy earth. I think the best twist on this is that they had a conflict between the engineeers (science) and another group, whether equal and internal (warrior clan) or engineered, external replicas of some sort that rebelled and tried to kill their creator. I also think that they used the black goo on several worlds, in one version it created the xenomorph, in another it created us. To them, perhaps, we are the same as the xenomorph, who would destroy them in similar fashion. So that was why they were trying to destroy us with their own biological weapons, although that might be too intricate. Clearly focusing on the themes is the solution: fathers and children, the Greek theme of Prometheus, of stealing fire (the first engineer creating life on earth and man becoming capable of such worldly destruction) and the relationship with death and the eternal question of the source of life, must be explored. It is, however, very human to ask WHY? That is the question David cannot understand, and one of the themes used as well.

Engineers never expected humans to come anywhere closer to their technology. They are afraid that if human race is left as it is humans might conquer the Engineer's world in future. So they plan to kill entire human race using a biological weapon (parasite plague)

Maybe the engineers that we saw weren't the true engineers but slaves/robots. The real engineers in fact look identical to us. The slaves/robots were built to be bigger so they have the necessary strength to carry out difficult tasks. The engineer we saw at the beginning was a slave/robot.

Then 2000 years ago there was a revolt by the slaves against their masters. Once the universe was populated by many Earth-like planets supporting humans. The slaves rebelled and destroyed these populations. Only one Earth remains. That is us.