My analysis of the slime:
The black slime is meant to deconstruct DNA. It takes it apart in order to create opportunities for new life. When it breaks DNA, that DNA can take on new forms (that's important). The first Engineer seen drinks the slime in order to spread his DNA throughout the planet, and be a gardener of life. Once the DNA is spread, it begins to form new life in a rapidly adapting state. Those forms are able to adapt so rapidly to ensure the successful spread of life, and can do so in the same way shown in the beginning of the movie.
The black slime kept in containers on LV-223 is a "weaponized" form of the slime; a quantity large enough to destroy a world's worth of DNA. It is used to erase creations that go awry. It is launched at a world and destroys all of the DNA contained on that world.
Holloway, upon infection, begins to break apart as the Engineer did, albeit slowly due to the tiny dose. As his DNA breaks down, he spreads it via insemination to Shaw. The broken-down DNA begins to rapidly adapt to its environment -- Shaw's womb.
This is why the slime reacts differently to different "host DNA". It is simply disassembled DNA being reconstructed. The mealworms become extremely strong snake-like creatures; Fifield doesn't ingest it, but its contact with him changes him drastically.
The slime that occupies Shaw's womb is simply the progenitor to the Xenomorph. The lifeform that is created by the slime's reaction with Holloway's DNA is one that is near the pinnacle of life's many forms -- in Bishop's words, "The perfect organism." It is a form that can reproduce using a host to evolve rapidly, absorbing traits in the host DNA.
TL;DR: the slime deconstructs DNA so it can reconstruct new lifeforms, and Shaw is impregnated with the result of one of these breakdowns. This resulting lifeform is the progenitor of the Xenomorph.
My analysis of why the Engineers had meant to destroy Earth's life and how it led to the events in Prometheus:
The Engineers spread and monitored life. They wished to spread life, for whatever reason. Once that life was spread, they watched the lifeform and attempted to guide their development.
On Earth, an Engineer sacrificed himself to sow his DNA as a seed throughout the "gardens" of the galaxies. This DNA eventually took the form of humans.
Throughout our development, they visited Earth to guide us. On those visits, they directed primitive man to a solar system where we might contact the Engineers.
In that solar system, the Engineers terraformed a moon for us to travel to. Once there, we might contact them.
Around 2000 years ago, human civilization became much less civilized. Ridley Scott chose not to use the crucifixion of Jesus as the catalyst, but something more like the rise of Rome and other empires/violent civilizations like it spurred the Engineers to decide that we were not fit to exist any longer. The human experiment was to be terminated -- the garden had too many weeds.
To eliminate us, they created a biological weapon using the DNA-destroying black slime that they used for gardening. They created this stockpile on a forward base as they knew it was dangerous and even threatened them as a species. They used the planet they had directed us to; I think this was because that base was the one assigned to monitor Earth and humankind.
They planned to drop this stockpile onto Earth to destroy mankind, but allow life to continue. The garden would be uprooted and new seeds would be sown.
Something triggered the stockpile -- perhaps something happened to an Engineer in the manufacturing process. Remember, all of the hologram Engineers were running out of the stockpile room. At the entrance to that room were multiple suits. I think that, upon entering the room, they would don their suits to avoid exposure to the slime. Something triggered the stockpile to rupture or overwhelm their suits.
The rupture caused the Engineers to run. None of them were able to escape the slime. As shown in the hologram, one made it fairly far away, but died. When he died, he collapsed onto the ground. When Shaw removed his helmet, it was clear that he had the DNA-destroying slime in his veins.
The lone surviving Engineer lived because he was in cryostasis. He was in that state in preparation for the journey to Earth. The stockpile ruptured as they made preparations to leave. Hence the Engineers running from the ship in full flight gear.
When Weyland wakes the Engineer, he awakens to find the creation he had set out to destroy. That creation asks him to preserve his already unnaturally long life. It was this hubris that had necessitated humanity's destruction, so he tries to complete his mission.
He first looks and smiles at David, seems to caress him, and then destroys him. I believe this is because he recognized David as humanity's feeble attempt to create life -- they could only manage an imitation. This seems almost touching to the Engineer. The apple, it would seem, does not fall far from the tree; humanity unwittingly followed in the Engineers' footsteps.
The Engineer kills the present crew and begins to set out for Earth. Not only was his mission 2000 years delayed, but the problem had compounded itself immensely. Not only had humans become self-centered and full of hubris, but they had actually sought out their creators to ask them for more than the life they had been given.
TL;DR: mankind was created as an experiment, but humanity became prideful and undeserving of the life they had been granted. The Engineers developed a weaponized form of their DNA-seeding material, and meant to use it to "reseed" Earth. In their effort, something went wrong and they were themselves affected by the weapon.
EDIT: Upon further investigation, I think this picture is extremely important. It depicts the Xenomorph's life cycle, from H.R. Giger. It seems to depict Engineers, in their exosuits, overseeing the birth of a facehugger and its subsequent infection of an Engineer as a host.
They may not be overseeing the birth cycle; instead, they might be happening upon the Xenomorph and recording the life cycle as they encountered it.
Either way, it proves that the Engineers had contact with the Xenomorph prior to the events in Prometheus. They knew about it and how it was born. I think the mural was made as a warning to anyone who encountered the Xenomorph, which was one of the clear and present dangers when the black slime was involved.
here is the pic