At the end of Prometheus, we are lead to believe that the Engineers were planning on using the black goo as a biological weapon to end human life on Earth. At the beginning of the movie, black goo is used to create the DNA that would lead to human life. Humans don't seem super aggressive compared to the life forms created by the black goo in the airlock room. Is this the same black goo used to create human life in the beginning of the movie, as the black goo that created the hyper-aggressive worm and the giant squid Dr. Shaw gave birth to? Or, did the Engineers try to "tweak" this goo to perform different actions on DNA?
This interview with director Ridley Scott and writer David Lindelof indicates that it is the same liquid, but their film is intentionally ambiguous about what exactly the liquid does.
For his part, Scott quips that the goo does three things: cleans your teeth, acts like Viagra, and causes a meltdown in the morning!