One of the basic human “needs” is the desire for simplicity. We have limited cognitive resources, and when we feel overwhelmed by complexity one adaptive strategy is to simplify things in our mind. This can be useful as long as we know we are oversimplifying. Problems arise when we mistake our schematic version for reality.
In this same vein we also like our narratives to be morally simple, so there is a tendency to replace the complex shades of gray with black and white. This is perhaps related to cognitive dissonance theory. We have a hard time reconciling how someone can be both good and bad, or how a good person can do bad things. So there is also a tendency to see people as all good or all bad. We can transcend these tendencies with maturity and wisdom, but that takes work.
A good example of the desire for simple moral clarity is the anti-vaccine movement. Their world is comprised of white hats and black hats (guess which one they perceive themselves as wearing), as evidenced by the blog posts and comments over at Age of Autism. There is a certain demand for purity of thought and message that seems to be getting worse over time in a self-reinforcing subculture. Many now see their struggle in apocalyptic terms.