plot explanation - What was the motivation behind the Engineers actions in Prometheus? - Movies & TV

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After watching href="http://www.imdb.com/title/tt1446714/">Prometheus I don't have a clear
idea of why the Engineers created the human beings. Because, if they created humans so
that they can test their bio-weapons on us, then it makes sense, but it's ambiguous.
Why did the Engineers want to destroy the human beings that
they have created?

Did I miss
something?

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class="normal">Answer

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.