Trying to think of a characteristic trait for someone that sees the world in black and white...almost like decisive / moralistic / ethical but not quite. Things are black and white, there's no gray area, something is either wrong or it's right. Their opinion on what is right vs wrong isn't necessarily correct (this person actually has a very skewed perspective on morality) but they stick to their own convictions.
He could well be a Moral Absolutist:
Moral absolutism is an ethical view that particular actions are intrinsically right or wrong. Stealing, for instance, might be considered to be always immoral, even if done for the well-being of others (e.g., stealing food to feed a starving family), and even if it does in the end promote such a good. Moral absolutism stands in contrast to other categories of normative ethical theories such as consequentialism, which holds that the morality (in the wide sense) of an act depends on the consequences or the context of the act. (-- Wikipedia)
Manichean means “duality,” so if you or your thinking is Manichean, you see things in black and white.
Manichean comes from the word Mani, which is the name of an apostle who lived in Mesopotamia in the 240’s, who taught a universal religion based on what we now call dualism.
If you believe in the Manichean idea of dualism, you tend to look at things as having two sides that are opposed. To Manicheans, life can be divided neatly between good or evil, light or dark, or love and hate. When you see Manichean, think “two.” -- source vocabulary.com
"The most crucial feature of neoconservatism is its Manichean worldview, wherein the Earth is pitted in an urgent struggle between purely good and purely evil nations. As George W. Bush famously told then Sen. Joe Biden: 'I don't do nuance.'" --- Jacob Bronsther; What Do Neocons Have to Do With Obama?; The Christian Science Monitor (Boston, Massachusetts); Sep 29, 2009.
Paul was living in an exciting Manichean world of friends and enemies, right and wrong - in which he was the chief protagonist. --- BBC Apr 27, 2015
I would suggest monochromatic thinker.
- to infer (a general principle) from particular facts or instances.
- to form (a general opinion or conclusion) from only a few facts or cases.
- to give a broad or general character or form to.
Random House Kernerman Webster's College Dictionary, © 2010
Granted, generalize doesn't specifically reference two opposing categories. But I think it's a very pertinent term for describing the sort of individuals who reach very generalized conclusions about the world, rather than seeing the complexities and specifics involved. Black and white thinking is a very basic form of generalization.
- A division into two contrasting parts or categories
American Heritage® Dictionary of the English Language, Fifth Edition.
Not a singular word, but still a term that means the exact same thing while condensing it into two words, rather than four.
naive /naɪˈiːv/ adjective –dictionary.com
lacking developed powers of analysis, reasoning, or criticism: a naive argument
It's naive to think that all things are black and white, and that one way is always either right or wrong. "Only the Sith deal in absolutes", is a very naive thing to say.
"but they stick to their own convictions."
steadfast: resolutely or dutifully firm and unwavering. –Google
(according to dictionaries and my spellcheck, it's steadfastly, but that word rolls off my tongue into a brick wall)
Your steadfastingly naive friend is resolute, dutifully firm and unwavering in their desire to continue lacking development in their powers of analysis, reasoning and criticism.
I sometimes describe people like that as rote.