So much of extremism is rooted in an overemphasis of the left hemisphere of the brain. It's the side rooted in right/wrong, us/them, analytics, etc. It's the ego side of the brain. The right hemisphere is where we hold abstract ideas, creativity, and empathy. When people are so far on the left hemisphere, one of the best approaches is to ease them into abstractions and personalizing things in order to shift from a label to a human being. The idea being to show that life isn't a nice simple right/wrong concrete thing, but is messy and full of unclear choices. Keep doing this and people start to question the concreteness of it all. It takes time though and building a relationship with someone, which is really difficult when you are talking about someone who is most concerned with right/wrong.