Maybe its the use of the term evil that causes problems. evil is concept that is even more mutable than truth, varying on social and moral norms.  I would accept a statement that exposure to extremists influences has the ability to affect the opinions and actions of otherwise moderate people. I cannot accept a oversimplification of this issue into a good v evil debate.  once you describe someone as evil you dehumanize them, which lessens ones ability to feel empathy and realize any insights which may come from such a change of perspective.