Blade Runner is one of those films that can only truly be made once in a lifetime, with a story as beautiful as its imagery. It's based on a Phillip K. Dick book called Do Androids Dream of Electric Sheep?, and though the core story is very similar, they do deviate a bit in certain themes, so whenever we deal with either one I'll make a clear distinction. The trailer for the sequel has just been released, so in honor of that, we'll go through the existence and behavior of the replicants. 

In Blade Runner, there are normal human beings, and there are replicants, which are genetically modified superhumans with a very short life span, and implanted memories, which have been banished from Earth. To deal with them, there are Runners, who are special private eyes dedicated to identifying and hunting down the replicants. They do this through the Voight-Kampff test, which asks questions that require genuine empathy to answer, and measures things like pupil dilation in response to certain questions to gauge subconscious reactions. When someone doesn't have empathy, either he's a true sociopath, or he's a replicant, and the difference between the two can be noticed when they answer the strange questions in the test. For example:

What scared people about the replicants was the fact that they were insanely strong, fast, better in any way than humanity except for their very short lifespan, and their lack of empathy combined with their superiority was, frankly, quite intimidating, but as the film goes along, you go to realize that what these people want is to be left alone with their memories until they die. That's not to say that they can't be "evil", in a way; they tend to use people to reach their goals and will not hesitate to kill anyone should it be more convenient to them, which is a side effect or direct result of their lack of empathy. 

Ridley Scott loved the ambiguity that could be used for Deckard himself, the best Runner that there is. Throughout the film, with certain imagery, it's implied that he's a replicant, and depending on whichever of the versions you watch, it's almost either confirmed or denied. There are also other characters who you can't really tell are replicants or not until either they reveal it or Deckard does, and though they do have the potential for danger, the Runner comes to realize that if left to their own devices they don't really have an incentive to destroy humanity or whatever it was that society feared of them. 

This is the argument that severely hindered racism throughout history and all the smaller phobias that society had, basically realizing that the others are humans as well and have the potential to either do good or evil, depending on how they were treated. Granted, the replicants aren't quite human, but they're pretty close. 

A lot of people tend to limit themselves because they fear that where they come from isn't good enough, but you have to realize that it's not where you're from that matters, but what you actually do, so you might as well live the best life you can. There's no conspiracy in your favor like Coelho would have you believe, but there's no conspiracy against you either, so don't look backwards from where you came. Rather, look towards the future. 

Do yourself a favor. Watch Blade Runner.