Q:

Is there a word for 'a person who can't differentiate between right and wrong'?

What would we call a person who cannot differentiate between right and wrong? 
I want to use this in my prose in a context as:

IMO, people who think Kim Jong Un is a great leader are [don't want to say stupid or unintelligent].

What can be used here?

A:

The word amoral may be what you want, but people who act amorally may be able to differentiate between what others call right and wrong, but they do not recognize any worth in the distinction and act accordingly.  For a term that means someone who who does not recognize the difference between right and wrong, we can go back to the somewhat-archaic term moral imbecile.  Such persons need not be intellectually impaired, but they are incapable of intellectually understanding other's distinction between good and evil and unable to have any emotional connection to the concepts.  A clinical description may be found in Mental Deficiency (amentia) by A F Tredgold (1922).  

A:

From your example, it sounds like you are talking not about someone who is incapable of morality, but rather someone who lacks judgment about someone else's morality. Such a person could be described as naive ( those who are picky about diacritics would render it naïve, note the two dots above the 'i' ).

A:

People who can't differentiate between right and wrong (have no moral compass) are amoral
Google Definition:

adjective
lacking a moral sense; unconcerned with the rightness or wrongness of something.

