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

After
watching 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.