Finally a good movie to spur me back into reviews and thoughts. Movies nowadays are a bit too cliche and direct, so the usual reviews make no sense and sounds irrelevant, because I'm no pro. However each and everyone of us is entitled to our own points and views, so for a movie like Prometheus, it is what you think or what they make you think which is more important.
Prometheus is a movie, which you say is science fiction, maybe science (if it could be real!). Now to those who aren't familiar or knew NOTHING about the Greek myths, this movie is named Prometheus not because the ship's name is Prometheus (well partly you can say, but no one names a movie after a ship ;] ).
According to Greek myths, Prometheus is a titan (think Clash/Wrath of the Titans, but not that big bad guy). He is known as a champion of mankind, who was famously known for his theft of fire for the human use. Prometheus stole heavenly fire from Heaven and gave it to humans, enabling the progress of civilization (we didn't rub wood with stone till we made our own fire! And cooking food with fire is a symbol or proof of civilization XD).
Well that is roughly what the myths say about him. In Western classical tradition, Prometheus is a symbol of human striving, particularly for SCIENTIFIC KNOWLEDGE, and the risk of overreaching or unintended consequences (quoted from Wikipedia). This guy, basically wanted to give equal footing for humans with Gods!
Now going back to the movie (SPOILER ALERT: DO NOT READ IF YOU HAVEN'T WATCH). Elizabeth Shaw and Charlie Holloway discovered similarities of star map in different civilizations, hence decided to prove that humans are not created by God, claimed by the religion, rather we could be a race faction of present aliens in other world - which they called "Engineers". This claim has been rubber stamped right at the start of the movie, where we see a humanoid alien drinking some gooey black liquid, disintegrating, leading to change of DNA, mutation, and in the end, new life forms-cells are formed and mitosis occurs.
The act, of traveling to the location designated by the star map, to prove that God never created humans, to put equal footing that it was actually man that created man, is an act of "Prometheusim". Hence, finally we now justify, why the movie is called Prometheus, where it isn't about Greek myths, nor has this guy appeared in the whole movie.
*I am an Atheist myself, so this theory or believe rather suits well with me XP.
Now questions and answers, based on my self interpretations:
1. Why did the Engineers created us and then wanted to destroy us?
Good question. In fact the one's everyone has in their minds. Even got me thinking straight away after I finished the movie. There are of course a lot of answers, since this is a very open-ended question. One of the perspectives is: Remember the beginning scene where we saw that Engineer drinking the black liquid? At that time the setting was on Earth. He sees a spaceship leaving, disrobe, drank the liquid, went into seizures, disintegrates, falls down the waterfall, his DNA destroying-but not entirely, and then live cells forming and multiplying. Theory one could be that he was indeed chosen to create us. Other theories might be a bit absurd, because they did came back and helped us in different civilizations, judging that different civilizations showed the same star map. So they couldn't be evil. So that leaves us to the next question.
2. Why would they want to destroy us?
This is an even harder question. Basically this is the question that fuels speculation of a 2nd movie. Even till the end of the first movie, even Elizabeth herself is clueless. So what is the reason, is up to your own imagination. As rich as possible. Maybe something happened back in their home planet, which could mean purging us to take over our planet. Maybe it is fun to create something just because you can, and destroying them too (what Charlie said to David). You can give a thousand reasons, but until a possible 2nd movie comes out, everything is possible.
3. Why are there so many different type of aliens? Why some mutate and some don't?
Remember there were 2 scientists that got lost. One got killed by a snake like (or cobra like) alien. One got contaminated by the black liquid. Think back. When Elizabeth, David and Charlie entered the chamber with the BIG engineer head statue, after lifting their foot from the ground, there is a scene where earthworms are seen wriggling in the soil. And then black liquid starts to ooze out from the vases, possibly mutating the worms, making them into the snake alien. This could be the same black liquid drank by the engineer at the start of the movie. Which is also the one that was spiked into Charlie's drink by David, passed into Elizabeth's body, making her 3 months pregnant with an alien baby. Note that again, the baby from Elizabeth is squid like. After somehow "mating" with the Engineer at the end, then only we see the typical Alien emerging from the dead Engineer body. Perhaps it shouldn't be a question, rather the laws of mutation huh?
4. So in the end, what is THE Alien?
Again if you don't know science, you'll get a hard time to understand. Elizabeth is sterile- meaning she can't have babies. But it wasn't clear how was she sterile. There are many reasons- failure for the fetus to adhere to the wall of the uterus, failure to produce an ovum and so on. So depending on what are the reasons that made Elizabeth sterile, what makes up the Alien varies. It can be the black liquid itself, or it could be a mutated sperm from Charlie (*snicker), it could even be the black liquid passed and combined with Elizabeth's ovum, if the reason of her sterility was due to the fetus failure to adhere to the uterus wall. Then again, it's again imagination.
So one could go on and on the speculate and think of the reasons and possible answers to the questions above. I've had enough myself though. Time to scoot off to studies.