(CNN) Extremism has a knack for metastasizing and coming back to wreak havoc upon its hosts. Once a cohort or society builds a hateful mindset, the hatred takes on a life of its own. Extremist ideology not only hurts a society’s enemies, but also eventually attacks from within and harms the society from which it originated. I first started observing this phenomenon abroad — and it saddens but does not surprise me that we are starting to see it haunt American democracy too.